the founding father of experimental atomic physics

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Otto Stern (1888-1969): The founding father of experimental atomic physics

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May 1932: Chadwick Reports the Discovery of the Neutron

By 1920, physicists knew that most of the mass of the atom was located in a nucleus at its center, and that this central core contained protons. In May 1932 James Chadwick announced that the core also contained a new uncharged particle, which he called the neutron.

Chadwick was born in1891 in Manchester, England. He was a shy child from a working class family, but his talents caught his teachers’ attention, and he was sent to study physics at the University of Manchester, where he worked with Ernest Rutherford on various radioactivity studies.

In 1914, Chadwick decided to travel to Germany to study with Hans Geiger. Unfortunately, not long after he arrived, WWI broke out and Chadwick ended up spending the next four years in a prison camp there. This did not entirely stop his scientific studies. To keep from being bored, he and some fellow prisoners formed a science club, lectured to each other, and managed to convince the guards to let them set up a small lab. Though many chemicals were hard to get hold of, Chadwick even found a type of radioactive toothpaste that was on the market in Germany at the time, and managed to persuade the guards to supply him with it. Using some tin foil and wood he built an electroscope and did some simple experiments.

After the war, Chadwick returned to England, where he finished his PhD in Cambridge in 1921 with Rutherford, who was then Director of Cambridge University’s Cavendish laboratory. Chadwick was able to continue to work on radioactivity, now with more sophisticated apparatus than tin foil and toothpaste. In 1923, Chadwick was appointed assistant director of Cavendish Laboratory.

Photo: AIP Emilio Segre Visual Archives

James Chadwick

Rutherford had discovered the atomic nucleus in 1911, and had observed the proton in 1919. However, it seemed there must be something in the nucleus in addition to protons. For instance, helium was known to have an atomic number of 2 but a mass number of 4. Some scientists thought there were additional protons in the nucleus, along with an equal number of electrons to cancel out the additional charge. In 1920, Rutherford proposed that an electron and a proton could actually combine to form a new, neutral particle, but there was no real evidence for this, and the proposed neutral particle would be difficult to detect.

Chadwick went on to work on other projects, but kept thinking about the problem. Around 1930, several researchers, including German physicist Walter Bothe and his student Becker had begun bombarding beryllium with alpha particles from a polonium source and studying the radiation emitted by the beryllium as a result. Some scientists thought this highly penetrating radiation emitted by the beryllium consisted of high energy photons. Chadwick had noticed some odd features of this radiation, and began to think it might instead consist of neutral particles such as those Rutherford had proposed.

One experiment in particular caught his attention: Frédéric and Irène Joliot-Curie had studied the then-unidentified radiation from beryllium as it hit a paraffin wax target. They found that this radiation knocked loose protons from hydrogen atoms in that target, and those protons recoiled with very high velocity.

Joliot-Curie believed the radiation hitting the paraffin target must be high energy gamma photons, but Chadwick thought that explanation didn’t fit. Photons, having no mass, wouldn’t knock loose particles as heavy as protons from the target, he reasoned. In 1932, he tried similar experiments himself, and became convinced that the radiation ejected by the beryllium was in fact a neutral particle about the mass of a proton. He also tried other targets in addition to the paraffin wax, including helium, nitrogen, and lithium, which helped him determine that the mass of the new particle was just slightly more than the mass of the proton.

Chadwick also noted that because the neutrons had no charge, they penetrated much further into a target than protons would.

In February 1932, after experimenting for only about two weeks, Chadwick published a paper titled “The Possible Existence of a Neutron,” in which he proposed that the evidence favored the neutron rather than the gamma ray photons as the correct interpretation of the mysterious radiation. Then a few months later, in May 1932, Chadwick submitted the more definite paper titled “The Existence of a Neutron.”

By 1934 it had been established that the newly discovered neutron was in fact a new fundamental particle, not a proton and an electron bound together as Rutherford had originally suggested.

The discovery of neutron quickly changed scientists’ view of the atom, and Chadwick was awarded the Nobel Prize in 1935 for the discovery. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei. It didn’t take long for scientists to find that hitting uranium with neutrons resulted in the fission of the uranium nucleus and the release of incredible amounts of energy, making possible nuclear weapons. Chadwick, whose discovery of the neutron had paved the way for the atomic bomb, worked on the Manhattan Project during WWII. He died in 1974.

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An Homage to Otto Stern

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• Dudley Herschbach 3 , 4  

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This chapter outlines an International Symposium held at Frankfurt on 1–5 September 2019. It marked the centennial of quantitative experiments with molecular beams, pioneered by Otto Stern. The European Physical Society declared Stern’s original laboratory a Historic Site, the fifth in Germany. As a graduate student in 1955, I learned about Otto Stern (1888–1969) and the impact of his molecular beams on quantum physics. I was intrigued and undertook crossed-beam experiments at Berkeley. In 1960 Otto came to a seminar that I gave. Later I met him, and heard some of his stories. The rest of the chapter describes his Nobel Prize and other Fests. In 1958 his long-term colleague, Immanuel Estermann, organized a celebration and Festschrift for Otto’s 70th birthday. In 1988, as a guest editor, I organized a Festschift for the centennial of Otto’s birth. That year, the German Physical Society established the Stern-Gerlach Prize as its highest award for experimental physics. Bretislav Friedrich and I wrote three papers about Stern. Since 2000, Horst Schmidt-Böcking at Frankfurt and colleagues have produced historical articles, along with a book about Otto, edited and bound all of his research papers into books, and diligently pursued letters to and from Otto, collecting them into large volumes.

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Rekindling of de Broglie–Bohm Pilot Wave Theory in the Late Twentieth Century: A Personal Account

A Confluence of Ideas and Experiments—A Tribute to Professor Walter Greiner

1 the frankfurt conference.

The joyful voluntary for trumpet [ 1 ] and organ made for a wonderful start for the Otto Stern Conference on 2 September 2019 in Alte Aula at the University of Frankfurt [ 2 ]. Professors Horst Schmidt-Böcking (Frankfurt) and Bretislav Friedrich (Berlin) were the Organizers; they developed a festive conference with a hefty booklet. As elders, J. Peter Toennies (Göttingen) and I (Harvard) were glad to be Honorary Chairs. About 140 participants were engaged in talks and discussions over three days. The first session of the Conference focused on history, marking the centennial of experiments with molecular beams launched by Otto Stern (Fig.  1 ). A dozen other sessions highlighted current areas of modern physics and chemistry. On the second day, Stern’s original laboratory was declared a European Physical Society Historic Site, the fifth in Germany. The ceremony included a keynote lecture, along with superb music [ 3 ], and unveiling of a plaque (Fig.  2 ) honoring the key discoveries made during 1919–1922 at Frankfurt. Most iconic was the experiment by Stern and Walther Gerlach that proved the reality of space quantization, thereby contributing decisively to the development of quantum mechanics.

Photo of Otto Stern during his Frankfurt time, circa 1920; courtesy of Alan Templeton, grandnephew of Otto Stern

Plaque that marks the European Physical Society Historic Site honoring the Physics Department at Frankfurt, dedicated on 3 September 2019, the fifth such in Germany

The Conference booklet [ 4 ] had two historical articles. One is titled “ Stern and Gerlach: How a Bad Cigar Helped Reorient Atomic Physics ,” by B. Friedrich and D. Herschbach (Physics Today, 2003 [ 5 ]). The second article, extensive and titled “ Otto Stern (1888 – 1969): The founding father of experimental atomic physics ,” by J. P. Toennies, H. Schmidt-Böcking, B. Friedrich, and J. C. A. Lower (Ann. Phys., 2011 [ 6 ]). The booklet articles had some festive aspects, suited for Otto. Along with his cigar, he liked amusements, movies, music, dancing, dining and travel by ship. At the Conference dinner, held in the Dorint Oberursel, Professor Ludger Wöste (Berlin) exhibited (Fig.  3 ) some of his Physical Amusements , fascinating and charming toys [ 7 ]. More fun came with a post-conference event, on September 5. A bus from Frankfurt took us to Geisenheim, for a boat ride on the Rhein to Braubach and back, accompanied with a wind ensemble, lively hornblowers!

Photo of Ludger Wöste, exhibiting one of his Physical Amusements

The Conference booklet also mentions that Otto Stern had a heyday period at the University of Hamburg (1923–1933), but Stern was forced by the Nazi regime to emigrate. He settled in the United States, first in Pittsburg at the Carnegie Institute (1933–1945) and then in Berkeley (1946–1969). He became a U.S. citizen in 1939 which enabled him to serve as a consultant in some military research projects. After the Second World War, Stern was generously helping many of his friends and colleagues with CARE packages. And he would not miss an opportunity to visit Europe to see his friends at conferences and meetings, in particular in Copenhagen, London, and foremost, in Zurich.

2 Learning About Otto Stern and Molecular Beams

In the spring of 1955, as a student at Stanford University, I took a course on statistical thermodynamics taught by a physics professor, Walter Meyerhof (1922–2006). In a brief digression, less than 5 min, he described Stern’s first beam experiment done in 1919 at Frankfurt to test the Maxwell-Boltzmann velocity distribution. Meyerhof had to emigrate (Fig.  4 ) in the Nazi era, and barely escaped the Gestapo [ 8 ]. Otherwise, it is likely he would not have been in a Stanford classroom, captivating a susceptible student. For me, learning about molecular beams was love at first sight. I remember a flush of excitement at the thought that this was the way to study elementary chemical reactions. Only five years later my own first beam apparatus was functioning at Berkeley and I had met Otto Stern himself.

Photo of Walter Meyerhof, on face of his book In the Shadow of Love – Stories from My Life (Fithian Press, 2002)

Meanwhile, my mentor at Stanford, Harold Johnston (1920–2012), had imbued me with his passion for chemical kinetics. It seemed to me a fundamental thing to try to understand how reactions occur at the molecular level. I wanted to find out what molecules are really doing, making and breaking bonds, instead of the gross macroscopic way that chemists were limited to before, trying to unravel many elementary steps at the same time. Hearing about Otto Stern, I thought by using molecular beams, you can really find out whether or not a reaction occurs as an elementary step. I immediately contacted Hal Johnston, with the naïve notion that chemical reactions could be studied by crossing two such molecular beams in a vacuum to isolate single collisions and directly detect the products. Hal laughed and said, “Well, sure, of course, but there’s not enough intensity.” It looked difficult. Molecular beam methods had found many applications in physics, but as of the early 1950s, very little had been done in chemistry.

In the fall of 1955 I learned more about Otto, when I moved to Harvard as a graduate student, aiming to obtain a Ph.D. in chemical physics. By golly, Norman Ramsey had just completed his book, titled Molecular Beams (Fig.  5 ). Ramsey gave a sparkling course, handing out the galley proofs. His excellent book reviewed the essence of Stern’s work and covered a wealth of further experimental and theoretical methods that produced many important discoveries. Early in Ramsey’s course, he discussed Stern’s velocity analysis study and actually announced, in his booming voice: “This would be a wonderful way to do chemistry!”

Photo of Norman Ramsey’s book, Molecular Beams (Oxford Press, 1956)

Ramsey also described the career of his mentor, Isidor Rabi, who made epochal molecular beam contributions to physics. Rabi had worked in Stern’s lab at Hamburg in 1927–1929 as a postdoctoral fellow before joining the physics faculty at Columbia. There he gladly displayed in his office a photo of Stern (Fig. 6 ) that he took in the early 1960s. In 1938, Rabi invented a versatile new beam instrument, delivering radiofrequency spectroscopy with extremely high resolving power. In October of 1955, Rabi was invited to give a special lecture at Harvard Physics. His title was “Science and the Humanities.” I was intrigued and still am. A friend, John Rigden, wrote a superb book: Rabi, Scientist and Citizen (Fig.  7 ).

Photo of Otto Stern in his early 70s that I.I. Rabi kept on display in his office at Columbia University

Photo of book of Rabi, Scientist and Citizen , by author John S. Rigden (Basic Books, New York, 1987)

In the chemistry department, an ebullient young instructor, William Klemperer, invited me to help build a high-temperature microwave spectrometer. This led us to study ionization of alkali atoms as a function of the surface temperature. Ramsey kindly lent us one of his beam machines over the Christmas vacation in 1956. This was a key episode for both Bill and me. He too fell in love with molecular beams, and immediately undertook to build an electric resonance beam apparatus. Bill and his students developed that into a cornucopia for molecular spectroscopy, unprecedented in resolution and chemical scope [ 9 ].

3 Meeting Otto Stern and Hearing Stories from Him

In the summer of 1959, I joined the chemistry faculty at the University of California at Berkeley as an assistant professor. With two graduate students, George Kwei and Jim Norris, we built a rudimentary crossed-beam apparatus that enabled us to measure the angular distributions for reactants and products. Our first reaction was K + CH 3 I → KI + CH 3 . In the fall of 1960, the physics department invited me to give a seminar about our work. In presenting the seminar, I naturally began with homage to Otto Stern, writing his name on the blackboard and sketching his velocity analysis and magnetic deflection experiments. During my seminar, I was surprised that two of the professors in the first row were engaged in animated conversation and swiveling around to look back at the audience. After the seminar, one of them asked me, “Did you know Otto Stern was in the audience?” Actually, I had noticed a fellow seated by himself, many rows up and back at left. In size and dark attire, he resembled Charlie Chaplin.

A meeting was arranged so that researchers using molecular beams at Berkeley could meet him. That was a week or so after the seminar. Professors Howard Shugart and William Nierenberg gathered a group of more than a dozen graduate students and postdoctoral fellows, systematically measuring spins and magnetic moments of radioactive nuclei using the Rabi molecular beam magnetic resonance method. George Kwei and Jim Norris came along with me. At the meeting, supplied with coffee, tea, and cookies, Stern at first seemed very shy. Soon, however, in response to questions, he began telling stories with gleeful verve. Six of them I have retold often.

In his velocity analysis experiment, the results were in approximate agreement with the Maxwell-Boltzmann distribution, as anticipated, but deviated from it in a systematic way. After sending off a paper, Stern received a letter pointing out that he should have included an additional factor of v, the velocity, that enters because the detected atoms must pass through a slit. That amendment improved the agreement with theory. After explaining this, Stern laughed heartily as he added: “That letter came from Albert Einstein!”

He spoke happily about his gratitude to Max Born, who was renowned as a fine speaker and raised money to build Stern’s apparatus at Frankfurt by giving public lectures.

With wry humor, Stern recalled that when he began teaching a physics course, he found it necessary to work late into the night preparing his lectures. He got into the habit of drinking strong black coffee to stay awake. Since then, he had found he could not fall asleep unless he first had a cup of such coffee.

The birth of the celebrated Stern-Gerlach experiment was told by Stern this way [ 10 ]: “The question whether a gas might be magnetically birefringent (in the words we used in those days) was raised at a seminar. The next morning I woke early, too early to go to the lab. As it was too cold to get out of bed, I lay there thinking about the seminar question and had the idea for the experiment.”

Stern said when he got to the lab, “I recruited Gerlach as a collaborator. He was a skillful experimentalist, and I was not. In fact, each part of the apparatus that I constructed had to be remade by Gerlach.” Cheerfully, Stern also said: “We were never able to get the apparatus to work before midnight.”

Stern’s “cigar story” was my favorite. As I remember, he told it with relish: “When finally all seemed to function properly, we had a strange experience. After venting to release the vacuum, Gerlach removed the detector flange. But he could see no trace of the silver atom beam and handed the flange to me. With Gerlach looking over my shoulder as I peered closely at the plate, we were surprised to see gradually emerge two distinct traces of the beam. Several times we repeated the experiment, with the same mysterious results. Finally we realized what it was. I smoked cheap cigars. These had a lot of sulfur in them, so my breath on the plate turned the silver into silver sulfide, which is jet black so easily visible. It was like developing a photographic film.”

This meeting with Stern lasted about two hours, whereas his cigar episode happened about four decades earlier. Another four decades came ahead: a new Center for Experimental Physics at the University of Frankfurt was dedicated in February 2002 to be named in honor of Stern and Gerlach. At the dedication, I expected to tell Stern’s cigar story, having told it many times over forty years. However, historical sleuthing by Bretislav Friedrich showed that two major aspects of my version of the cigar story were wrong. The cigar episode must have occurred at an earlier stage, because Stern was away in Rostock. When Gerlach had finally resolved a pair of distinct traces and by then he was using a photographic development process. The occasion of the Frankfurt dedication prompted Bretislav and me to carry out an experimental test. We found that bad breath did not suffice, although when cigar smoke is exhaled directly onto the deposition plate, the silver traces did rapidly become visible.

I had hoped to meet Stern again at a seminar. But I didn’t have the sense to ask Shugart to invite Stern again. In 1963 my group and lab moved to Harvard; alas, I failed to invite him there.

4 Fests with Otto Stern Present

With the Stern-Gerlach experiment, Stern had acquired fame and liked to visit other countries. In 1930 he lectured for some weeks at the University of California at Berkeley and was awarded an honorary degree of L.L.D. On the way there, during December 1929, he met Ernest Lawrence on coincident visits to Harvard. Unaccustomed to Prohibition, Stern asked Lawrence to take him to a speak-easy. While contemplating the circular rings left by their wine glasses, Lawrence diagrammed an idea he had been mulling over for months, a means to accelerate ions in a magnetic field. Stern urged him to stop talking about it, get back to his lab at Berkeley, and work on the idea. Lawrence took the advice and soon developed his cyclotron [ 11 ]. As early as 1931, Stern reported in Europe with great enthusiasm on the future of the cyclotron. However, when Stern was forced to emigrate in 1933, he did not receive an offer from Berkeley.

Otto likely enjoyed a fine cigar on December 10, 1944. The Nobel Prizes broke the five-year respite owing the Second World War; no prizes were awarded from 1940 until 1944. The Swedish Academy made up part of the loss by naming the 1943 winners along with those for 1944. The 1943 prize for physics went to Otto Stern and the 1944 prize to Isidor Rabi [ 12 ]. They couldn’t go to Sweden—the war was still on—so the ceremony was held in New York, at the Waldorf-Astoria (Figs.  8 and 9 ). Rabi said: “It was an enormous pleasure and an excuse for many parties …” At the parties, a little ditty was sung with the refrain: “Twinkle, twinkle Otto Stern/How did Rabi so much learn?” Otto did come to Stockholm for the 1946 Nobel celebration, and he delivered his Les Prix Nobel lecture, only 7 pages [ 13 ].

Photo, courtesy Diana Templeton Killen

Otto Stern’s Nobel Document .

Courtesy Diana Templeton Killen

The Swedish ambassador Eric Boström presents the Nobel awards in physics to Stern (left) and Rabi (middle) at the New York Waldorf Astoria Hotel on December 10, 1944.

In 1958, a Festschrift was held for Stern’s 70th birthday, organized by Immanuel Estermann (1900–1973). A long-term colleague, Estermann obtained his doctorate in 1921 at Hamburg, and began working with Otto, first at Rostock, then at Hamburg. When forced to emigrate in 1933, he was hired by the Carnegie Institute (now Carnegie-Mellon University) at Pittsburgh alongside with Otto. During the Second World War, Immanuel worked first on Radar and then transferred to the Manhattan Project. After Otto retired to Berkeley in 1945, Estermann left Pittsburgh in 1950 to join the Office of Naval Research. He also became editor of the series of Advances in Atomic and Molecular Physics.

Estermann edited a book: Recent Research in Molecular Beams (Fig.  10 ), a collection of ten chapters dedicated to Otto Stern (Academic Press, 1959) [ 14 ]. Estermann wrote the first chapter about the historic work in Hamburg (1922–1933). The other chapters describe fresh research among seven institutions. Only one dealt with chemistry. Sheldon Datz and Ellison Taylor, at Oak Ridge National Laboratory, in 1955 had published a crossed molecular beam reaction, K + HBr → KBr + H. It made an impact on eager physical chemists. By 1965, a Gordon Research Conference in New Hampshire was accepted. A lively group of 60 graduate students and mentors were discussing theory and experiments for reactions with molecular beams. When I mentioned Otto Stern, a shout came from Sheldon Datz: “For all of us, he is our Father.” Of course, I responded: “Otto is a bachelor.” There was a roar: “We are all bastards!” Since then, dynamics of molecular reactive collisions has flourished, with conferences every two years or so for more than 50 years.

Photo of book Recent Research in Molecular Beams , A collection of papers dedicated to Otto Stern on the occasion of his 70th birthday ; edited by Immanuel Estermann (Academic Press, 1959). Table of Contents displayed

In 1961, Otto Stern had an oral interview, by Res Jost [ 15 ]. Also, in 1962, Immanuel Estermann had an extensive oral history interview by John L. Heilbron [ 16 ]. Immanuel was engaged in writing a book on the History of the Molecular Beam Method when he died in 1973. A paper in 1975 was published in Am. J. Phys. [ 17 ] covering the essence of the first two chapters (edited by S. N. Foner) on the important evolutionary period, 1919–1933. It contains some amusing historical sidelights on the research personalities that dominated that period.

In 1973 Emilio Segrè (1905–1982) delivered a biographical memoir of Otto Stern for the National Academy of Sciences [ 18 ]. Segrè had worked with Otto Stern and Otto Frisch (1904–1979) during 1931–1933 at Hamburg on space quantization. When Otto Stern retired to Berkeley, Emilo was on the faculty, so they often met. During his last years, Otto remained interested in discoveries in particle physics and astrophysics. A few days before his death, Otto argued vehemently about enormous energy output of quasars and was dissatisfied that astrophysicists rejected his interpretation! Emilio and many others count Otto Stern among the greatest physicists of the twentieth century.

5 Centennial of Otto Stern and Beyond

In 1987, after writing a long article, Molecular Dynamics of Elementary Chemical Reactions [ 19 ], I felt attention was deserved in 1988, to have a Festschrift for the centennial of Stern’s birth. In his Hamburg era, 1923–1933, Stern had inaugurated a series of papers which he called Untersuchungen zur Molekularstrahlmethode (U.z.M.) published in Zeitschrift für Physik. The series reached 30 papers. That journal fifty years later had grown to four categories. So I urged the Editor in Chief, Ingolf V. Hertel, to produce a centennial issue. He asked me to do it as a Guest Editor for Z. Phys. D Atoms, Molecules and Clusters (Fig.  11 ). Here are parts of the Preface, An homage to Otto Stern:

Festschrift in memoriam Otto Stern on the 100th anniversary of his birth: Zeitschrift für Physik D, Atoms, Molecules and Clusters 10 , 109–392, June 1988 (Springer International); with six samples among the 31 articles

His legacy abides in many domains of physics, but especially in vigorous progeny exemplifying his favorite theme: “the characteristic simplicity and directness of the molecular ray method.” Concepts and techniques developed by Stern have proved remarkably durable and versatile, yet still more vital for science is his exemplary pursuit of insight and beauty.

Next comes a reprint of Otto’s 1921 paper (plus an English translation); it proposed “an experiment which, if successful, will decide unequivocally between the quantum and classical views.” A list of his publications follows—only 60 (Stern’s total was 71, including publications in nonscientific venues). Then come reminiscences of Stern by I. I. Rabi as told to John Rigden, some in the last days before Rabi’s death (11 January 1988). A review of Stern’s development of molecular beams was given by Norman Ramsey, from his lecture presented at a convocation in Hamburg commemorating Stern (4 February, 1988). Ramsey provided a list [ 20 ] of 32 major “advances that contributed to physics from the field of molecular beams … during the past seventy years.”

The Festschrift indeed had 31 exceptional papers, largely from Stern’s kindred spirits. Here are six samples (Fig.  11 ). Among them are a “continuous Stern-Gerlach effect” that glimpses the primordial Big Bang. Or is “spin coherence like Humpty-Dumpty?” Also, a liquid jet. Or an Otto Stern double bank shot. Or using an electrospray source that generates molecular beams of huge proteins.

Also in 1988 the German Physical Society established the Stern-Gerlach Prize. In 1993 the Prize became the Stern-Gerlach Medal. It is awarded for excellence in experimental physics, in parallel with the existing Max Planck Medal for excellence in theory.

Hamburg also had in 1988 an Otto Stern Symposium, as noted, with Norman Ramsey. A two-day Stern event was held in 2013 with many speakers. This is available on YouTube . A single-day Stern event was held in 2018.

In 1998 Bretislav Friedrich and I contributed to an unusual event: Science in Culture , held in Proceedings of the American Academy of Arts and Science, Cambridge, Massachusetts [ 21 ]. The event was dedicated to Gerald Holton, an outstanding historian of science, for his studies of Einstein. Bretislav and I delivered a sizeable paper titled: Space Quantization: Otto Stern’s Lucky Star [ 21 ]. We hoped to make it accessible to anyone with only vague memories of high-school science, and to induce chuckles rather than growls.

During December 11–14, 2000 there was held in Berlin a Quantum Theory Centenary, celebrating the famous talk of Max Planck. Fifty scientists were invited to present reviews of their fields to a large international audience. The proceedings were collected as a Festschrift in the “Annalen der Physik”. I was asked to talk about Otto Stern and molecular beams, before 1935. That led to five decisive episodes: discovery of space quantization; de Broglie matter waves; anomalous magnetic moments of the proton and neutron; recoil of an atom of emission of a photon; and the limitation of scattering cross-sections for molecular collisions imposed by the uncertainty principle [ 22 ]. The Centenary Symposium was splendid, having quantum entanglement and teleportation, discovery of quarks, quantum cosmology and more!

In 2002, when the Stern-Gerlach Center for Experimental Physics at Frankfurt was named, a memorial plaque (Fig.  12 ) was mounted near the entrance of the building where the Stern-Gerlach experiment took place. Horst Schmidt-Böcking had a major role in the installation of the SGE plaque and much more. At the 2019 Conference, the plaque was moved near the room where the SGE was done. The inscription, in translation reads: “ In February 1922 … was made the fundamental discovery of space quantization of the magnetic moments of atoms. The Stern - Gerlach Experiment is the basis of important scientific and technological developments in the 20th century, such as nuclear magnetic resonance, atomic clocks, or lasers … ”

A memorial plaque honoring Otto Stern and Walther Gerlach was mounted in February 2002 next to the entrance of the building where the S-G experiment took place 80 years earlier

Frankfurt was busy well before the 2019 Conference. In 2005, Wolfgang Trageser collected papers [ 23 ] to form a Stern - Stunden book (Fig.  13 ). In 2011, Horst produced with Karin Reich [ 24 ] an Otto Stern book (Fig.  14 ). He wrote historical articles [ 25 ] with others (2011, 2016) and edited all of Otto’s research papers [ 26 ] into books (Fig.  15 ). Moreover, Horst with Alan Templeton and Wolfgang Trageser were extraordinarily diligent in pursuing letters to and from Otto, organizing and collecting them into large volumes [ 27 ] (Fig.  16 ).

Photo of book by W. Trageser, ed., Stern - Stunden Höhepunkte Frankfurter Physik , comprised of collected articles. A sampling was made from [ 5 ] and [ 21 ], pp. 149–170

Photo of book by H. Schmidt-Böcking and K. Reich, Otto Stern : Physiker, Querdenker, Nobelpreistrager (Frankfurt/Main: Societäts-Verlag, 2011)

H. Schmidt-Böcking, K. Reich, A. Templeton, W. Trageser, V. Vill, eds., Otto Sterns Veröffentlichungen — Band 1, Sterns Veröffentlichungen 1912 bis 1916 (Springer Spektrum, 2016)

H. Schmidt-Böcking, A. Templeton, W. Trageser, eds., Otto Sterns gesammelte Briefe — Band 1, Hochschullaufbahn und die Zeit des Nationalsozialismus (Springer Spektrum, 2018)

When I visited Berkeley again, to give a Commencement address in 2012, Alan took me to the Chemistry Library to see Otto’s magnificent desk that he had donated to the library (Fig.  17 ).

Dudley with Alan Templeton, visiting Otto Stern’s desk, now in the Chemistry Library at the University of California, Berkeley

The 2019 Conference aimed to show that many key areas of modern physics and chemistry originated in the seminal molecular beam work of Otto Stern and his colleagues. The sessions highlighted the state of the art: foundations of quantum mechanics, as well the problems of quantum measurement; magnetic and electronic resonance spectroscopy, including magnetic resonance imaging and its medical applications; high-precision measurements; cold atoms and molecules; reaction dynamics; matter-wave scattering; magneto-optical traps and optical lattices; and exotic beams, among microdroplet chemistry, liquid beams, and helium droplet beams.

Beyond the history session, memories of Otto and his colleagues endure. Alan Templeton gave a festive talk: My uncle Otto Stern . Other presenters were Peter Toennies: Otto Stern and Wave - Particle Duality ; Dan Kleppner: Our Patrimony from Otto Stern and My Memories of Otto Frisch ; Karl von Meyenn: Stern’s Friendship with Wolfgang Pauli ; and Horst: Stern’s Relation to Gerlach .

Concluding my introduction to the Conference, I offered a song by Cole Porter, “ Experiment ,” more than 80 years old [ 28 ].

This is the closing paragraph of Otto Stern’s Nobel Lecture [ 13 ]:

The most distinctive characteristic property of the molecular ray method is its simplicity and directness. It enables us to make measurements on isolated neutral atoms or molecules with macroscopic tools. For this reason, it is especially valuable for testing and demonstrating directly fundamental assumptions of the theory.

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-5754, U.S.A. https://chemistry.harvard.edu/people/[email protected]

Musikalische Umrahmung at the Opening of the Otto Stern Fest; the musicians were: Wolfgang Huhn, trumpet; Karsten Schwind, organ

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Cole Porter (1891–1964) song lyrics of “Experiment” (1931) are available at http://www.songlyrics.com/cole-porter/experiment-lyrics/

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Acknowledgements

For helpful information, I am grateful to Horst, to Bretislav, to Peter, to Alan, to Diana Templeton-Killen, to my wife Georgene, and to Cole Porter.

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Correspondence to Dudley Herschbach .

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Bretislav Friedrich

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Horst Schmidt-Böcking

Appendix: A Historical Puzzle

Recently, I learned from Eugene Wigner (1902–1995) that in the 1920s Michael Polanyi (1891–1976) had an original idea to use molecular beams [ 29 ]. At Haber’s Institute in Berlin-Dahlem, he was famed for chemical kinetics, using a diffusion flame method involving sodium vapor, halogens, and organic halides. He must have known about the celebrated molecular beams used by Otto Stern at Frankfurt and Hamburg, not so far from Dahlem. Searches in the archives of correspondence of both turned up only one letter from Stern to Polanyi. It is dated 10 October 1928, but with questions unrelated to beams or reactions.

I first met Michael Polanyi in 1962 when he came to Berkeley to deliver a series of lectures on the philosophy of science. He also visited my lab and observed a molecular beam experiment. On other occasions, especially at a Faraday Discussion in London in 1973, Michael heard about many beam results, but didn’t mention that he had once intended to try beams. In 1962, I missed an opportunity to arrange for Michael Polanyi to meet and exchange stories with Otto Stern.

Appendix: Lyrics of Cole Porter’s “Experiment”

Before you leave these portals

To meet less fortunate mortals

There’s just one final message

I would give to you

You all have learned reliance

On the sacred teachings of science

So I hope, through life you never will decline

In spite of philistine

To do what all good scientists do

Make it your motto day and night

And it will lead you to the light

The apple on the top of the tree

Is never too high to achieve

So take an example from Eve

Though interfering friends may frown

Get furious

At each attempt to hold you down

If this advice you’ll only employ

The future can offer you infinite joy

And merriment

And you’ll see

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Herschbach, D. (2021). An Homage to Otto Stern. In: Friedrich, B., Schmidt-Böcking, H. (eds) Molecular Beams in Physics and Chemistry. Springer, Cham. https://doi.org/10.1007/978-3-030-63963-1_1

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Copenhagen Interpretation of Quantum Mechanics

As the theory of the atom, quantum mechanics is perhaps the most successful theory in the history of science. It enables physicists, chemists, and technicians to calculate and predict the outcome of a vast number of experiments and to create new and advanced technology based on the insight into the behavior of atomic objects. But it is also a theory that challenges our imagination. It seems to violate some fundamental principles of classical physics, principles that eventually have become a part of western common sense since the rise of the modern worldview in the Renaissance. The aim of any metaphysical interpretation of quantum mechanics is to account for these violations.

The Copenhagen interpretation was the first general attempt to understand the world of atoms as this is represented by quantum mechanics. The founding father was mainly the Danish physicist Niels Bohr, but also Werner Heisenberg, Max Born and other physicists made important contributions to the overall understanding of the atomic world that is associated with the name of the capital of Denmark.

In fact Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics, and neither of them ever used the term “the Copenhagen interpretation” as a joint name for their ideas. In fact, Bohr once distanced himself from what he considered to be Heisenberg’s more subjective interpretation ( APHK , p. 51). The term is rather a label introduced by people opposing Bohr’s idea of complementarity, to identify what they saw as the common features behind the Bohr-Heisenberg interpretation as it emerged in the late 1920s. Today the Copenhagen interpretation is mostly regarded as synonymous with indeterminism, Bohr’s correspondence principle, Born’s statistical interpretation of the wave function, and Bohr’s complementarity interpretation of certain atomic phenomena.

1. The Background

2. classical physics, 3. the correspondence rule, 4. complementarity, 5. the use of classical concepts, 6. the interpretation of the quantum formalism, 7. misunderstandings of complementarity, 8. the divergent views, 9. the measurement problem, 10. complementarity, decoherence, and probabilities, 11. new perspectives, references to work by bohr, other references, other internet resources, related entries.

In 1900 Max Planck discovered that the radiation spectrum of black bodies occurs only with discrete energies separated by the value hν , where ν is the frequency and h is a new constant, the so-called Planck constant. According to classical physics, the intensity of this continuous radiation would grow unlimitedly with growing frequencies, resulting in what was called the ultraviolet catastrophe. But Planck’s suggestion was that if black bodies only exchange energy with the radiation field in a proportion equal to hν that problem would disappear. The fact that the absorption and the emission of energy is discontinuous is in conflict with the principles of classical physics. A few years later Albert Einstein used this discovery in his explanation of the photoelectric effect. He suggested that light waves were quantized, and that the amount of energy which each quantum of light could deliver to the electrons of the cathode, was exactly hν . The next step came in 1911 when Ernest Rutherford performed some experiments shooting alpha particles into a gold foil. Based on these results he could set up a model of the atom in which the atom consisted of a heavy nucleus with a positive charge surrounded by negatively charged electrons like a small solar system. Also this model was in conflict with the laws of classical physics. According to classical mechanics and electrodynamics, one might expect that the electrons orbiting around a positively charged nucleus would continuously emit radiation so that the nucleus would quickly swallow the electrons.

At this point Niels Bohr entered the scene and soon became the leading physicist on atoms. In 1913 Bohr, visiting Rutherford in Manchester, put forward a mathematical model of the atom which provided the first theoretical support for Rutherford’s model and could explain the emission spectrum of the hydrogen atom (the Balmer series). The theory was based on two postulates:

• An atomic system is only stable in a certain set of states, called stationary states, each state being associated with a discrete energy, and every change of energy corresponds to a complete transition from one state to another.
• The possibility for the atom to absorb and emit radiation is determined by a law according to which the energy of the radiation is given by the energy difference between two stationary states being equal to hν .

Some features of Bohr’s semi-classical model were indeed very strange compared to the principles of classical physics. It introduced an element of discontinuity and indeterminism foreign to classical mechanics:

• Apparently not every point in space was accessible to an electron moving around a hydrogen nucleus. An electron moved in classical orbits, but during its transition from one orbit to another it was at no definite place between these orbits. Thus, an electron could only be in its ground state (the orbit of lowest energy) or an excited state (if an impact of another particle had forced it to leave its ground state.)
• It was impossible to predict when the transition would take place and how it would take place. Moreover, there were no external (or internal) causes that determined the “jump” back again. Any excited electron might in principle move spontaneously to either a lower state or down to the ground state.
• Rutherford pointed out that if, as Bohr did, one postulates that the frequency of light ν , which an electron emits in a transition, depends on the difference between the initial energy level and the final energy level, it appears as if the electron must “know” to what final energy level it is heading in order to emit light with the right frequency.
• Einstein made another strange observation. He was curious to know in which direction the photon decided to move off from the electron.

Between 1913 and 1925 Bohr, Arnold Sommerfeld and others were able to improve Bohr’s model, and together with the introduction of spin and Wolfgang Pauli’s exclusion principle it gave a reasonably good description of the basic chemical elements. The model ran into problems, nonetheless, when one tried to apply it to spectra other than that of hydrogen. So there was a general feeling among all leading physicists that Bohr’s model had to be replaced by a more radical theory. In 1925 Werner Heisenberg, at that time Bohr’s assistant in Copenhagen, laid down the basic principles of a complete quantum mechanics. In his new matrix theory he replaced classical commuting variables with non-commuting ones. The following year, Erwin Schrödinger gave a simpler formulation of the theory in which he introduced a second-order differential equation for a wave function. He himself attempted a largely classical interpretation of the wave function. However, already the same year Max Born proposed a consistent statistical interpretation in which the square of the absolute value of this wave function expresses a probability density for the outcome of a measurement.

Bohr saw quantum mechanics as a generalization of classical physics although it violates some of the basic ontological principles on which classical physics rests. Some of these principles are:

• Physical objects (systems of objects) exist in space and time and physical processes take place in space and time, i.e., it is a fundamental feature of all changes and movements of physical objects (systems of objects) that they happen on a background of space and time;
• Physical objects (systems) are localizable, i.e., they do not exist everywhere in space and time; rather, they are confined to definite places and times;
• A particular place can only be occupied by one object of the same kind at a time;
• Two physical objects of the same kind exist separately; i.e., two objects that belong to the same kind cannot have identical location at an identical time and must therefore be separated in space and time;
• Physical objects are countable, i.e., two alluded objects of the same kind count numerically as one if both share identical location at a time and counts numerically as two if they occupy different locations at a time;
• The principle of separated properties , i.e., two objects (systems) separated in space and time have each independent inherent states or properties;
• The principle of value determinateness , i.e., all inherent states or properties have a specific value or magnitude independent of the value or magnitude of other properties;
• The principle of causality , i.e., every event, every change of a system, has a cause;
• The principle of determination , i.e., every later state of a system is uniquely determined by any earlier state;
• The principle of continuity , i.e., all processes exhibiting a difference between the initial and the final state have to go through every possible intervening state; in other words, the evolution of a system is an unbroken path through its state space; and finally
• The principle of the conservation of energy , i.e., the energy of a closed system can be transformed into various forms but is never gained, lost or destroyed.

Due to these principles it is possible within, say, classical mechanics, to define a state of a system at any later time with respect to a state at any earlier time. So whenever we know the initial state consisting of the system’s position and momentum, and know all external forces acting on it, we also know what will be its later states. The knowledge of the initial state is usually acquired by observing the state properties of the system at the time selected as the initial moment. Furthermore, the observation of a system does not affect its later behavior or, if observation somehow should influence this behavior, it is always possible to incorporate the effect into the prediction of the system’s later state. Thus, in classical physics we can always draw a sharp distinction between the state of the measuring instrument being used on a system and the state of the physical system itself. It means that the physical description of the system is objective because the definition of any later state is not dependent on measuring conditions or other observational conditions.

Much of Kant’s philosophy can be seen as an attempt to provide satisfactory philosophical grounds for the objective basis of Newton’s mechanics against Humean scepticism. Kant thus argued that classical mechanics is in accordance with the transcendental conditions for objective knowledge. Kant’s philosophy undoubtedly influenced Bohr in various ways, as many scholars in recent years have noticed (Hooker 1972; Folse 1985; Honner 1987; Faye 1991; Kaiser 1992; and Chevalley 1994). Bohr was definitely neither a subjectivist nor a positivist philosopher, as Karl Popper (1967) and Mario Bunge (1967) have claimed. He explicitly rejected the idea that the experimental outcome is due to the observer. As he said: “It is certainly not possible for the observer to influence the events which may appear under the conditions he has arranged” ( APHK , p. 51). Not unlike Kant, Bohr thought that we could have objective knowledge only in case we can distinguish between the experiential subject and the experienced object. It is a precondition for the knowledge of a phenomenon as being something distinct from the sensorial subject, that we can refer to it as an object without involving the subject’s experience of the object. In order to separate the object from the subject itself, the experiential subject must be able to distinguish between the form and the content of his or her experiences. This is possible only if the subject uses causal and spatial-temporal concepts for describing the sensorial content, placing phenomena in causal connection in space and time, since it is the causal space-time description of our perceptions that constitutes the criterion of reality for them. Bohr therefore believed that what gives us the possibility of talking about an object and an objectively existing reality is the application of those necessary concepts, and that the physical equivalents of “space,” “time,” “causation,” and “continuity” were the concepts “position,” “time,” “momentum,” and “energy,” which he referred to as the classical concepts . He also believed that the above basic concepts exist already as preconditions of unambiguous and meaningful communication, built in as rules of our ordinary language. So, in Bohr’s opinion the conditions for an objective description of nature given by the concepts of classical physics were merely a refinement of the preconditions of human knowledge.

The guiding principle behind Bohr’s and later Heisenberg’s work in the development of a consistent theory of atoms was the correspondence rule. The full rule states that a transition between stationary states is allowed if, and only if, there is a corresponding harmonic component in the classical motion ( CW Vol. 3 , p. 479). Bohr furthermore realized that according to his theory of the hydrogen atom, the frequencies of radiation due to the electron’s transition between stationary states with high quantum numbers, i.e. states far from the ground state, coincide approximately with the results of classical electrodynamics. Hence in the search for a theory of quantum mechanics it became a methodological requirement to Bohr that any further theory of the atom should predict values in domains of high quantum numbers that should be a close approximation to the values of classical physics. The correspondence rule was a heuristic principle meant to make sure that in areas where the influence of Planck’s constant could be neglected the numerical values predicted by such a theory should be the same as if they were predicted by classical radiation theory.

The Bohr-Sommerfeld core model of the atomic structure came into trouble in the beginning of the 1920s due to the fact that it couldn’t handle an increasing number of spectroscopic phenomena. In 1924 Wolfgang Pauli introduced a new degree of freedom according to which two electrons with the same known quantum numbers could not be in the same state. A year later, in 1925, Ralph Kronig, Georg Uhlenbeck and Samuel Goudsmit explained this new degree of freedom by introducing the non-classical concept of electron spin. It has been suggested, however, that Pauli’s proposal meant a lethal blow not only to the Bohr-Sommerfeld model, but also to the correspondence principle because “how to reconcile the classical periodic motions presupposed by the correspondence principle with the classically non-describable Zweideutigkeit of the electron’s angular momentum?” (Massimi 2005, p. 73)

Although the exclusion rule and the introduction of spin broke with the attempt to explain the structure of the basic elements along the lines of the correspondence argument (as Pauli pointed out in a letter to Bohr) Bohr continued to think of it as an important methodological principle in the attempt to establish a coherent quantum theory. In fact, he repeatedly expressed the opinion that Heisenberg’s matrix mechanics came to light under the guidance of this very principle. In his Faraday Lectures from 1932, for instance, Bohr emphasizes: “A fundamental step towards the establishing of a proper quantum mechanics was taken in 1925 by Heisenberg who showed how to replace the ordinary kinematical concepts, in the spirit of the correspondence argument, by symbols referring to the elementary processes and the probability of their occurrence” ( CC , p. 48). Bohr acknowledged, however, that the correspondence argument failed too in those cases where particular non-classical concepts have to be introduced into the description of atoms. But he still thought that the correspondence argument was indispensable for both structural and semantic reasons in constructing a proper quantum theory as a generalised theory from classical mechanics.

Indeed, spin is a quantum property of the electrons which cannot be understood as a classical angular momentum. Needless to say, Bohr fully understood that. But he didn’t think that this discovery ruled out the use of the correspondence rule as guidance to finding a satisfactory quantum theory. A lengthy quotation from Bohr’s paper “The Causality Problem in Atomic Physics” (1938) gives evidence for this:

Indeed, as adequate as the quantum postulates are in the phenomenological description of the atomic reactions, as indispensable are the basic concepts of mechanics and electrodynamics for the specification of atomic structures and for the definition of fundamental properties of the agencies with which they react. Far from being a temporary compromise in this dilemma, the recourse to essentially statistical considerations is our only conceivable means of arriving at a generalization of the customary way of description sufficiently wide to account for the features of individuality expressed by the quantum postulates and reducing to classical theory in the limiting case where all actions involved in the analysis of the phenomena are large compared with a single quantum. In the search for the formulation of such a generalization, our only guide has just been the so called correspondence argument, which gives expression for the exigency of upholding the use of classical concepts to the largest possible extent compatible with the quantum postulates. ( CC , p. 96)

This shows that, according to Bohr, quantum mechanics, as formulated by Heisenberg, was a rational generalization of classical mechanics when the quantum of action and the spin property were taken into account.

The correspondence rule was an important methodological principle. In the beginning it had mostly a clear technical meaning for Bohr as a principle of theory construction, but later he also recognized it as having a specific interpretative function. Apparently, Bohr’s use of the principle up to Heisenberg’s formulation of quantum mechanics was as a heuristic principle for how to generalize mathematically a coherent theory from classical mechanics, while afterwards he relied on it to support his understanding of the physical meaning of the new theory. In the first situation, the pre-QM, Bohr used the principle dynamically as a methodological principle for generating a new theory (Bokulich & Bokulich 2005). As a methodological principle, it guided physicists to how a coherent theory of quantum phenomena might be achieved, and as a methodological principle, it can be understood as an umbrella principle that applied to different aspects of a formal (syntactical) extension of classical physics depending on the context of discussion. In the second situation, post-QM, his appeal to the correspondence principle had more the character of a vindication of his own interpretation of quantum mechanics. In this post-QM situation, the principle achieved a semantic role in the sense that Bohr saw it as a consequence of the descriptive demand of using classical concepts, and thereby his physical interpretation of the QM formalism in relation to the theory’s description of experimental results.

It is obvious that it makes no sense to compare the numerical values of the theory of atoms with those of classical physics unless the meaning of the physical terms in both theories is commensurable. The correspondence rule was based on the epistemological idea that classical concepts were indispensable for our understanding of physical reality, and it is only when classical phenomena and quantum phenomena are described in terms of the same classical concepts that we can compare different physical experiences. It was this broader sense of the correspondence rule that Bohr often had in mind later on. He directly mentioned the relationship between the use of classical concepts and the correspondence principle in 1934 when he wrote in the Introduction to Atomic Theory and the Description of Nature :

[T]he necessity of making an extensive use … of the classical concepts, upon which depends ultimately the interpretation of all experience, gave rise to the formulation of the so-called correspondence principle which expresses our endeavours to utilize all the classical concepts by giving them a suitable quantum-theoretical re-interpretation. ( ATDN , p. 8)

Bohr’s practical methodology stands therefore in direct opposition to Thomas Kuhn and Paul Feyerabend’s historical view that succeeding theories, like classical mechanics and quantum mechanics, are incommensurable. In contrast to their philosophical claims of meaning gaps and partial lack of rationality in the choice between incommensurable theories, Bohr believed not just retrospectively that quantum mechanics was a natural generalization of classical physics, but he and Heisenberg followed in practice the requirements of the correspondence rule. Thus, in the mind of Bohr, the meaning of the classical concepts did not change but their application was restricted. This was the lesson of complementarity.

After Heisenberg had managed to formulate a consistent quantum mechanics in 1925, both he and Bohr began their struggle to find a coherent interpretation for the mathematical formalism. Heisenberg and Bohr followed somewhat different approaches (Camilleri 2009). Where Heisenberg looked to the formalism and developed his famous uncertainty principle or indeterminacy relation, Bohr chose to analyze concrete experimental arrangements, especially the double-slit experiment. In a way Bohr merely regarded Heisenberg’s relation as an expression of his general notion that our understanding of atomic phenomena builds on complementary descriptions. At Como in 1927 he presented for the first time his ideas according to which certain different descriptions are said to be complementary. Bohr never gave a precise definition of complementarity; however, Simon Saunders (2005) offers a formal reconstruction of complementarity in modern terminology.

Bohr pointed to two sets of descriptions which he took to be complementary. On the one hand, there are those that attribute either kinematic or dynamic properties to the atom; that is, “space-time descriptions” are complementary to “claims of causality”, where Bohr interpreted the causal claims in physics in terms of the conservation of energy and momentum. On the other hand, there are those descriptions that ascribe either wave or particle properties to a single object. How these two kinds of complementary sets of descriptions are related is something Bohr never indicated (Murdoch 1987). Even among people, like Rosenfeld and Pais, who claimed to speak on behalf of Bohr, there is no agreement. The fact is that the description of light as either particles or waves was already a classical dilemma, which not even Einstein’s definition of a photon really solved since the momentum of the photon as a particle depends on the frequency of the light as a wave. Furthermore, Bohr eventually realized that the attribution of kinematic and dynamic properties to an object is complementary because the ascription of both of these conjugate variables rests on mutually exclusive experiments. The attribution of particle and wave properties to an object may, however, occur in a single experiment; for instance, in the double-slit experiment where the interference pattern consists of single dots. So within less than ten years after his Como lecture Bohr tacitly abandoned “wave-particle complementarity” in favor of the exclusivity of “kinematic-dynamic complementarity” (Held 1994).

It was clear to Bohr that any interpretation of the atomic world had to take into account an important empirical fact. The discovery of the quantization of action meant that quantum mechanics could not fulfill the above principles of classical physics. Every time we measure, say, an electron’s position, the apparatus and the electron interact in an uncontrollable way, so that we are unable to measure the electron’s momentum at the same time. Until the mid-1930s when Einstein, Podolsky and Rosen published their famous thought-experiment with the intention of showing that quantum mechanics was incomplete, Bohr spoke as if the measurement apparatus disturbed the electron. This paper had a significant influence on Bohr’s line of thought. Apparently, Bohr realized that speaking of disturbance seemed to indicate—as some of his opponents may have understood him—that atomic objects were classical particles with definite inherent kinematic and dynamic properties. After the EPR paper he stated quite clearly: “the whole situation in atomic physics deprives of all meaning such inherent attributes as the idealization of classical physics would ascribe to such objects.”

Hence, according to Bohr, the state of the measuring device and the state of the object cannot be separated from each other during a measurement but they form a dynamical whole. Bohr called this form of holism “the individuality” of the atomic process. Thereby, he had in mind not only that the interaction is uncontrollable but also that the system-cum-measurement forms an inseparable unity due to the entanglement – although Bohr’s did not use this term (Faye 1991, 1994; Howard 1994, 2004).

Also after the EPR paper Bohr spoke about Heisenberg’s “indeterminacy relation” as indicating the ontological consequences of his claim that kinematic and dynamic variables are ill-defined unless they refer to an experimental outcome. Earlier he had often called it Heisenberg’s “uncertainty relation”, as if it were a question of a merely epistemological limitation. Furthermore, Bohr no longer mentioned descriptions as being complementary, but rather phenomena or information. He introduced the definition of a “phenomenon” as requiring a complete description of the entire experimental arrangement, and he took a phenomenon to be a measurement of the values of either kinematic or dynamic properties.

Bohr’s more mature view, i.e., his view after the EPR paper, on complementarity and the interpretation of quantum mechanics may be summarized in the following points:

• The interpretation of a physical theory has to rely on an experimental practice.
• The experimental practice presupposes a certain pre-scientific practice of description, which establishes the norm for experimental measurement apparatus, and consequently what counts as scientific experience.
• Our pre-scientific practice of understanding our environment is an adaptation to the sense experience of separation, orientation, identification and reidentification over time of physical objects.
• This pre-scientific experience is grasped in terms of common categories like thing’s position and change of position, duration and change of duration, and the relation of cause and effect, terms and principles that are now parts of our common language.
• These common categories yield the preconditions for objective knowledge, and any description of nature has to use these concepts to be objective.
• The concepts of classical physics are merely exact specifications of the above categories.
• The classical concepts—and not classical physics itself—are therefore necessary in any description of physical experience in order to understand what we are doing and to be able to communicate our results to others, in particular in the description of quantum phenomena as they present themselves in experiments;
• Planck’s empirical discovery of the quantization of action requires a revision of the foundation for the use of classical concepts, because they are not all applicable at the same time. Their use is well defined only if they apply to experimental interactions in which the quantization of action can be regarded as negligible.
• In experimental cases where the quantization of action plays a significant role, the application of a classical concept does not refer to independent properties of the object; rather the ascription of either kinematic or dynamic properties to the object as it exists independently of a specific experimental interaction is ill-defined.
• The quantization of action demands a limitation of the use of classical concepts so that these concepts apply only to a phenomenon, which Bohr understood as the macroscopic manifestation of a measurement on the object, i.e. the uncontrollable interaction between the object and the apparatus.
• The quantum mechanical description of the object differs from the classical description of the measuring apparatus, and this requires that the object and the measuring device should be separated in the description, but the line of separation is not the one between macroscopic instruments and microscopic objects. It has been argued in detail (Howard 1994) that Bohr pointed out that parts of the measuring device may sometimes be treated as parts of the object in the quantum mechanical description.
• The quantum mechanical formalism does not provide physicists with a ‘pictorial’ representation: the ψ -function does not, as Schrödinger had hoped, represent a new kind of reality. Instead, as Born suggested, the square of the absolute value of the ψ -function expresses a probability density for the outcome of a measurement. Due to the fact that the wave equation involves an imaginary quantity this equation can have only a symbolic character, but the formalism may be used to predict the outcome of a measurement that establishes the conditions under which concepts like position, momentum, time and energy apply to the phenomena.
• The ascription of these classical concepts to the phenomena of measurements rely on the experimental context of the phenomena, so that the entire setup provides us with the defining conditions for the application of kinematic and dynamic concepts in the domain of quantum physics.
• Such phenomena are complementary in the sense that their manifestations depend on mutually exclusive measurements, but that the information gained through these various experiments exhausts all possible objective knowledge of the object.

Bohr thought of the atom as real. Atoms are neither heuristic nor logical constructions. A couple of times he emphasized this directly using arguments from experiments in a very similar way to Ian Hacking and Nancy Cartwright much later. What he did not believe was that the quantum mechanical formalism was true in the sense that it gave us a literal (‘pictorial’) rather than a symbolic representation of the quantum world. It makes much sense to characterize Bohr in modern terms as an entity realist who opposes theory realism (Folse 1986; Faye 1991).

It is because of the imaginary quantities in quantum mechanics (where the commutation rule for canonically conjugate variable, p and q , introduces Planck’s constant into the formalism by qp − pq = ih /2π that quantum mechanics does not give us a ‘pictorial’ representation of the world. Neither does the theory of relativity, Bohr argued, provide us with a literal representation, since the velocity of light is introduced with a factor of i in the definition of the fourth coordinate in a four-dimensional manifold ( CC , p. 86 and p. 105). Instead these theories can only be used symbolically to predict observations under well-defined conditions. Therefore, many philosophers have interpreted Bohr as an antirealist or an instrumentalist when it comes to theories. However, Bohr’s reference to the use of imaginary number in quantum mechanics as an argument for his rejection of a pictoral representation may seem misplaced. The use of imaginary numbers is more a question about the conventional choice of scale whether measurements should be represented in terms of imaginary or real number than an indication of a certain magnitude expressed in terms of these numbers is not real. Dieks (2017) gives a nuanced discussion of Bohr’s argument, and he concludes that in the context of quantum mechanics Bohr saw imaginary numbers to be associated with incompatible physical quantities.

In general, Bohr considered the demands of complementarity in quantum mechanics to be logically on a par with the requirements of relativity in the theory of relativity. He believed that both theories were a result of novel aspects of the observation problem, namely the fact that observation in physics is context-dependent. This again is due to the existence of a maximum velocity of propagation of all actions in the domain of relativity and a minimum of any action in the domain of quantum mechanics. And it is because of these universal limits that it is impossible in the theory of relativity to make an unambiguous separation between time and space without reference to the observer (the context) and impossible in quantum mechanics to make a sharp distinction between the behavior of the object and its interaction with the means of observation ( CC , p. 105).

Complementarity is first and foremost a semantic and epistemological reading of quantum mechanics that carries certain ontological implications. Bohr’s view was, to phrase it in a modern philosophical jargon, that the truth conditions of sentences ascribing a certain kinematic or dynamic value to an atomic object are dependent on the apparatus involved, in such a way that these truth conditions have to include reference to the experimental setup as well as the actual outcome of the experiment. This claim is called Bohr’s indefinability thesis (Murdoch 1987; Faye 1991). Hence, those physicists who accuse this interpretation of operating with a mysterious collapse of the wave function during measurements haven’t got it right. Bohr accepted the Born statistical interpretation because he believed that the ψ -function has only a symbolic meaning and does not represent anything real. It makes sense to talk about a collapse of the wave function only if, as Bohr put it, the ψ -function can be given a pictorial representation, something he strongly denied.

Indeed, Bohr, Heisenberg, and many other physicists considered complementarity to be the only rational interpretation of the quantum world. They thought that it gave us the understanding of atomic phenomena in accordance with the conditions for any physical description and the possible objective knowledge of the world. Bohr believed that atoms are real, but it remains a much debated point in recent literature what sort of reality he believed them to have, whether or not they are something beyond and different from what they are observed to be. Henry Folse argues that Bohr must operate with a distinction between a phenomenal and a transcendental object. The reason is that this is the only way it makes sense to talk about the physical disturbance of the atomic object by the measuring instrument as Bohr did for a while (Folse 1985, 1994). But Jan Faye has replied that Bohr gave up the disturbance metaphor in connection with his discussion of the EPR thought-experiment because he realized that it was misleading. Moreover, there is no further evidence in Bohr’s writings indicating that Bohr would attribute intrinsic and measurement-independent state properties to atomic objects (though quite unintelligible and inaccessible to us) in addition to the classical ones being manifested in measurement (Faye 1991).

A central element in the Copenhagen Interpretation is Bohr’s insistence on the use of classical concepts both with respect to describing experimental results and endowing quantum formalism with an empirical interpretation. The special cognitive status ascribed to the classical concepts is something Bohr stressed from the very beginning. Here is a quotation from 1934:

No more is it likely that the fundamental concepts of the classical theories will ever become superfluous for the description of physical experience. … It continues to be the application of these concepts alone that makes it possible to relate the symbolism of the quantum theory to the data of experience. ( ATDN , p. 16)

Later he expressed the same view in an often quoted passage:

It is decisive to recognize that, however far the phenomena transcend the scope of classical physical explanation, the account of all evidence must be expressed in classical terms. The argument is simply that by the word ‘experiment’ we refer to a situation where we can tell to others what we have done and what we have learned and that, therefore, the account of the experimental arrangement and of the results of the observations must be expressed in unambiguous language with suitable application of the terminology of classical physics. ( APHK , p. 39)

Bohr saw the classical concepts as necessary for procuring unambiguous communication about what happens in the laboratory. Classical concepts are indispensable, because they enable physicists to describe observations in a clear common language, and because they are the ones by which the physicists connect the mathematical formalism with observational content.

Over the years, different authors have come up with different explanations of why Bohr thought that classical concepts were unavoidable for the description of quantum phenomena. Here we shall group those explanations in relation to five different philosophical frameworks: 1) Empiricism, 2) Kantianism, 3) Pragmatism, 4) Darwinianism, and 5) Experimentalism.

Empiricism . This view is represented by the logical positivists. They believed that the interpretation of any scientific theory should be grounded in empirical observations. No theory, according the positivists, is cognitively meaningful unless its terms can be connected to terms that are able to express results that would verify that theory. Observational terms refer directly to observable things or observable properties of physical objects, whereas theoretical terms are explicitly defined by correspondence rules connecting them with the observational terms. Hence classical terms, like position and momentum, are exactly such terms that enable physicist to ascribe a physical meaning to quantum mechanics.

Kantianism . Many philosophers and physicists have recognized a strong kinship between Kant and Bohr’s thinking or a direct Kantian influence on Bohr. In the thirties C.F. von Weizsäcker and Grete Hermann attempt to understand complementarity in the light of neo-Kantian ideas. As von Weizsäcker puts it many years later, “The alliance between Kantians and physicists was premature in Kant’s time, and still is; in Bohr, we begin to perceive its possibility”. A series of modern scholars (Folse 1985; Honner 1982, 1987; Faye 1991; Kaiser 1992; Chevalley 1994; Pringe 2009; Cuffaro 2010; Bitbol 2013, 2017; and Kauark-Leite 2017) has also emphasized the Kantian parallels. Although these scholars find common themes, they also disagree to what extent Kantian or neo-Kantian ideas can be used as spectacles through which we may vision Bohr’s understanding of quantum mechanics. On the other hand, Cuffaro (2010) holds that any proper “interpretation of Bohr should start with Kant”, and that “complementarity follows naturally from a broadly Kantian epistemological framework.” Kant’s assumption was that our forms of intuition and our categories of thoughts constitute the transcendental conditions for the possibility of any objective experience. Thus, space and time are referred to as the forms of intuition, and the categories of understanding such as causation, unity, plurality, and totality are the a priori concepts which the mind imposes on the sense impressions that appear in our intuition. In a similar way, it is argued that Bohr saw concepts like space, time, causation, unity, and totality as a priori categories that was necessary for any objective description of quantum phenomena, and that classical physics was an explication and operationalization of these a priori concepts.

Pragmatism . Some scholars have advocated for a more pragmatic explanation of Bohr’s thesis concerning the indispensability of classical concepts. Here the interpretation focuses on how we experimentally get to know something about atoms. We find out about atoms by interacting with atomic systems, not by picturing them, and the interaction are accounted for in terms of experiential categories. The pragmatists typically reject the a priori status of the mind’s categories as they take them to be contingent. From a physical perspective it is a simple matter of facts that we need classical language to understand our scientific practise; it does not require any philosophical justification (Dieks 2017). Likewise, Dorato (2017) compares Bohr’s indispensable thesis to Peter Strawson’s descriptive metaphysics according to which we all share a common conceptual scheme about the experiential world which cannot be given a further justification. Also Folse notes, in a comparison between Bohr and I.C. Lewis, that classical concepts reflect our empirical needs and shared interests and may eventually change if these needs and interests change (Folse 2017). The common language together with the development of a physical clarification of some basic empirical concepts gave us the classical physics because such an improved language enables us to communicate in an unambiguous and objective manner about our observations. As Bohr puts it: “... even when the phenomena transcend the scope of classical physical theories, the account of the experimental arrangement and the recording of observations must be given in plain language, suitably supplemented by technical physical terminology. This is a clear logical demand, since the very word ”experiment“ refers to a situation where we can tell others what we have done and what we have learned.” ( APHK , p. 71). The use of classical concepts to grasp the world is beneficial for understanding each other. Such empirical concepts provide us with an objective description of the function and outcome of physical experiments.

Darwinism . In several places Bohr speaks about the classical concepts as embodied in our common language, which is adapted to account for our physical experiences. The selection of the word “adapted to” seems to indicate that Bohr relied on Darwin’s theory of natural selection in his search for an explanation. The classical concepts are indispensable for the description of our experience because we are forced by nature to use a common language that is adapted to reporting our visual experiences, which again is a result of humans’ adaptation to their physical environment (Faye, 2017). Apart from Bohr’s use of the word “adapted to”, Bohr’s former assistant Leon Rosenfelt, who was an ardent defender of Bohr’s complementarity, explicitly suggests that “the complementary logic” is due to human evolution: “I suspect the development of a computing and communication system like our brain demands about that complexity of organization which has been reached by our own species in the course of evolution” (Rosenfeld, (1961 [1979]), p. 515). Natural selection installs certain permanent visual cognitive schemes in our predecessors, and this cognitive adaptation explains why these schemes, later reflected in our common language, gain a privileged epistemic status, and keep this status in physics in terms of refined classical concepts.

Experimentalism . Camilleri (2017) calls Bohr the philosopher of experiment. Others such as Perovic (2013) have also suggested that Bohr was more occupied by understanding the outcome of quantum experiments than by interpreting the quantum formalism. This view is substantiated in Perovic (2021) where he points to traditional induction as Bohr’s methods of constructing hypotheses from experimental data, just as he argues that this fact is a prerequisite for understanding Bohr’s view on correspondence and complementarity. However, Camilleri proposes in his paper that the challenge Bohr was facing was that, on the one hand, experimental observation requires a sharp separation of the experiment and the observed object, and, on the other hand, because of what we today call entanglement, “it is no longer possible sharply to distinguish between the autonomous behaviour of a physical object and its inevitable interaction with other bodies serving as measuring instruments” ( CC , p. 84). So, according to Camilleri, Bohr solved this challenge by making a distinction between the function and the structure of an experiment.

Bohr’s central insight was that if a measuring instrument is to serve its purpose of furnishing us with knowledge of an object – that is to say, if it is to be described functionally – it must be described classically. Of course, it is always possible to represent the experimental apparatus from a purely structural point of view as a quantum-mechanical system without any reference to its function. However, any functional description of the experimental apparatus, in which it is treated as a means to an end, and not merely as a dynamical system, must make use of the concepts of classical physics. (Camilleri, 2017, pp. 30–31)

This analysis explains not only why Bohr thought that classical concepts were indispensable for interpretational purposes, but also indicates why he thought that properties like momentum, position, and duration could be attributed only to an atom object in relation to a specific experimental arrangement. As Dieks (2017) mentions while denying any deeper philosophical motivation on Bohr’s part: the use of classical concepts is part of the laboratory life. “This classical description is basically just the description in terms of everyday language, generalized by the addition of physics terminology, and it is the one we de facto use to describe our environment” (Dieks 2017). But because of quantum of action, symbolized by Planck’s constant, the function of experiments that supply the physicists with exact information about space-time coordinations is incompatible with experiments whose function it is to supply them with exact information about energy and momentum.

Indeed, there are both similarities and overlaps between some of the proposed explanations concerning the indispensability of classical concepts. Yet, not all of the suggested explanations can be true. Even though the aim of Bohr’s effort is to give an empirical interpretation of the quantum formalism, his empiricism is different from that of the logical positivists. He does not seek to reduce terms concerning theoretical entities to terms about sense-data or purely perceptual phenomena. He insists only that the empirical evidence physicists collect from their experiments on atomic objects has to be described in terms of the same concepts which were developed in classical mechanics in order for them to understand what the quantum theory is all about.

Nevertheless, the various explanations all give us some hints into the complexity of Bohr’s thinking concerning the description of physical experiments. At different times, he seems to put emphasis on one aspect rather than another, depending on the specific context of discussion. Sometimes he was occupied with the interpretation of experiments, sometimes with the relationship between actual experiments and the formulation of quantum mechanics. In emphasizing the necessity of classical concepts for the description of quantum phenomena, Bohr might have been influenced by Kantian-like ideas or neo-Kantianism (Hooker, 1994). But if so, he was a naturalized or a pragmatized Kantian. The classical concepts are merely explications of common-sense concepts that are already a result of our perceptual adaptation to the world. These concepts and the conditions of their application determine the conditions for objective knowledge. The discovery of the quantization of action has revealed to us, however, that we cannot apply these concepts to quantum objects as we did in classical physics. The use of classical concepts in the domain of quantum mechanics has to be restricted with respect to their use in classical mechanics. Now kinematic and dynamic properties (represented by conjugate variables) can be meaningfully ascribed to the object only in relation to some actual experimental results, whereas classical physics attributes such properties to the object regardless of whether we actually observe them or not. In other words, Bohr denied that classical concepts could be used to attribute properties to a physical world in-itself behind the perceptual phenomena, i.e. properties different from those being observed. In contrast, classical physics rests on an idealization, he said, in the sense that it assumes that the physical world has these properties in-itself, i.e. as inherent properties, independent of their actual observation.

Classical concepts serve the important function of connecting the quantum mechanical symbolism with experimental observations. If one accepts that Bohr’s grasp of physics began with his understanding of the role of physical experiments, this understanding had strong implications for his empirical interpretation of the quantum formalism. The modern scholarly debate has taken Bohr to be an instrumentalist, an objective anti-realist (Faye 1991), a phenomenological realist (Shomar 2008), or a realist of various sorts (Folse 1985, 1994; Favrholdt 1994; MacKinnon 1994; Howard 1994, 2004; Zinkernagel 2015, 2016). But very often the various participants do not give an exact specification of how they understand these terms and how these terms apply to Bohr’s thinking. The whole discussion becomes confused because different authors use terms like “realism” and “antirealism” differently in relation to Bohr. For instance, Faye (1991) holds that Bohr is an entity realist but a non-representationalist concerning theories. Therefore he calls Bohr an objective antirealist. In contrast, Folse (1986) who also sees Bohr as both a entity realist and a theoretical non-representationalist calls him a realist. Moreover, Bohr himself would probably refuse to put any such labels on his own view.

It is certain that Bohr regarded atomic objects as real ( ATDN , p. 93 and p. 103). Their existence has been confirmed by countless experiments. Hence, phrased in a modern terminology Bohr might be classified as an entity realist in the sense that experiments reveal their classical properties in relation to an experimental set-up. Such a view does not fit traditional instrumentalism where the introduction of unobservable entities is a logical construction in order to classify various empirical observations together. But entity realism corresponds with objective anti-realism, phenomenological realist, and all other forms of realism because it does not indicate anything about one’s attitude towards theories. A further issue is then how to interpret a physical theory. Does or doesn’t the quantum formalism, according to Bohr, represent the world over and above being a tool for prediction?

Here are four statements which seem to show that Bohr was an instrumentalist concerning scientific theories in general and the quantum formalism in particular.

• The purpose of scientific theories “is not to disclose the real essence of phenomena but only to track down, so far as it is possible, relations between the manifold aspects of experience” ( APHK , p. 71).
• “The ingenious formalism of quantum mechanics, which abandons pictorial representation and aims directly at a statistical account of quantum processes …” ( CC , p. 152).
• “The formalism thus defies pictorial representation and aims directly at prediction of observations appearing under well-defined conditions” (CC, p. 172).
• “The entire formalism is to be considered as a tool for deriving predictions of definite and statistical character …” ( CC , p. 144).

In these four statements Bohr mentions the absence of “pictorial representation” twice in relation to the quantum formalism. The term “pictorial representation” stands for a representation that helps us to visualize what it represents in contrast to “symbolic representation”. A pictorial representation is a formalism that has an isomorphic relation to the objects it represents such that the visualized structure of the representation corresponds to a similar structure in nature. Conversely, a symbolic representation does not stand for anything visualizable. It is an abstract tool whose function it is to calculate a result whenever this representation is applied to an experimental situation. With respect to the formalism of quantum mechanics it is particularly one’s interpretation of the wave function that determines whether one thinks of it symbolically as a tool for calculation of statistical outcomes or thinks of is as representing a real physical field.

In a close reading of the Como-paper, Dennis Dieks reaches the conclusion that “The notion that the lecture is meant to promulgate an instrumentalist interpretation of quantum theory according to which the whole formalism possesses only mathematical and no physical descriptive content is thus immediately seen to sit uneasily with the textual evidence.” (Dieks 2017, p. 305). In other words, Dieks goes against the more general interpretation of Bohr according to which Bohr only believed that the wave function formalism is a mere tool for prediction. Hans Halvorson (2019) expresses a similar view as Dieks does, arguing that even though Bohr used the word symbolic it should not be understood as if the wave function were “nonrepresentational” or “uninterpreted”. Halvorson thinks that we should read “symbolic” as philosophers do in the continental tradition going back to Kant, and he suggests with reference to Pringe (2014) that there might be parallels to Ernst Cassirer’s view on symbols. Just because Bohr writes off quantum formalism as a pictoral representation, it still gives us some insight into physical reality.

In the argument for his reading, Dieks points to another of Bohr’s argument against seeing Schödinger’s wave function as representing anything real. This argument concerns the fact that the wave function in quantum mechanics cannot represent a three-dimensional entity.

Bohr himself tells us that his second argument, about the dimensionality of configuration space, is the most important one: “ above all there can be no question of an immediate connexion with our ordinary conceptions because... the wave equation is associated with the so-called co-ordinate space.” In other words, the Schrödinger wave in the case of a many-particle system cannot be a physical wave in three-dimensional space (which would be an “ordinary conception”) since it “lives” in a high-dimensional mathematical space. (Dieks 2017, p. 308)

Then Dieks argues that even though this is an argument against wave function realism, it is not an argument that excludes the wave function from containing information about the quantum world. Dieks compares this argument to the one that denies phase space realism. “We can consistently deny the physical reality of phase space and still be realists with respect to particles. So we should not mistake Bohr’s argument for the symbolic character of the wave function for an argument in favor of instrumentalism tout court ” (Dieks 2017, p. 308). The difference between classical many-particles system placed in a phase space and a system of quantum objects placed in the configuration space is, however, that the description of many particles in phase space can be decomposed into a description of single particles in three-dimensional physical space, whereas the sum of the quantum waves associated with many particles in configuration space yields yet another superimposed quantum wave, which cannot be decomposed into a description of single particles in three-dimensional space. Dieks then continues to show how the structural features of the quantum formalism guided Bohr in his interpretation of quantum mechanism. Likewise, he argues that Bohr’s pronouncements on the meaning of quantum mechanics should first of all be seen as responses to concrete physical problems, rather than as expressions of a preconceived philosophical doctrine. His analysis results in a finding that Bohr’s qualitative interpretation is in line with modern non-collapse theories.

Complementarity has been commonly misunderstood in several ways, some of which shall be outlined in this section. First of all, earlier generations of philosophers and scientists have often accused Bohr’s interpretation of being positivistic or subjectivistic. Today philosophers have almost reached a consensus that it is neither. There are, as many have noticed, both typically realist as well as antirealist elements involved in it, and it has affinities with Kant or neo-Kantianism. The influence of Kant or Kantian thinking on Bohr’s philosophy seems to have several sources. Some have pointed to the German tradition from Hermann von Helmholtz (Chevalley 1991, 1994; Brock 2003); others have considered the Danish philosopher Harald Høffding to be the missing link to Kantianism (Faye 1991; Christiansen 2006; and Pringe 2020). A dissenting opinion is Favrholdt (1992).

But because Bohr’s view on complementarity has wrongly been associated with positivism and subjectivism, much confusion still seems to stick to the Copenhagen interpretation. Don Howard (2004) argues, however, that what is commonly known as the Copenhagen interpretation of quantum mechanics, regarded as representing a unitary Copenhagen point of view, differs significantly from Bohr’s complementarity interpretation. He holds that “the Copenhagen interpretation is an invention of the mid-1950s, for which Heisenberg is chiefly responsible, [and that] various other physicists and philosophers, including Bohm, Feyerabend, Hanson, and Popper, hav[e] further promoted the invention in the service of their own philosophical agendas” (p. 669).

More recently, Mara Beller (1999) argued that Bohr’s statements are intelligible only if we presume that he was a radical operationalist or a simple-minded positivist. In fact, complementarity was established as the orthodox interpretation of quantum mechanics in the 1930s, a time when positivism was prevalent in philosophy of science, and some commentators have taken the two to be closely associated. During the 1930s Bohr was also in touch with some of the leading neopositivists or logical empiricists such as Rudolph Carnap, Otto Neurath, Philip Frank, and the Danish philosopher Jørgen Jørgensen (Faye and Jaksland 2021a). Although their anti-metaphysical approach to science may have had some influence on Bohr (especially around 1935 during his final discussion with Einstein about the completeness of quantum mechanics), one must recall that Bohr always saw complementarity as a necessary response to the indeterministic description of quantum mechanics due to the quantum of action. The quantum of action was an empirical discovery, not a consequence of a certain epistemological theory, and Bohr thought that indeterminism was the price to pay to avoid paradoxes. Never did Bohr appeal to a verificationist theory of meaning; nor did he claim classical concepts to be operationally defined. But it cannot be denied that some of the logical empiricists rightly or wrongly found support for their own philosophy in Bohr’s interpretation and that Bohr sometimes confirmed them in their impressions (Faye 2008).

Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation. This may be true for people like Heisenberg. But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Only if one can interpret a quantum measurement as an interaction between an instrument and an object, whose state is literally represented by Schrödinger’s wave function, and therefore taken to contain all potential values of observation, does it make sense to claim that the measurement forces the object to manifest one of these potential vales. Indeed, such a literal interpretation of the state vector implies that these values are somehow intrinsically present in the object with a certain probability all at once. In contrast, Bohr believed that particular kinematical and dynamical properties are relational because their attribution to a quantum system makes sense only in relation to a particular experimental set-up and therefore that these numerical properties could have a specific value only during a measurement.

Third, Bohr flatly denied the ontological thesis that the subject has any direct impact on the outcome of a measurement. Hence, when he occasionally mentioned the subjective character of quantum phenomena and the difficulties of distinguishing the object from the subject in quantum mechanics, he did not think of it as a problem confined to the observation of atoms alone. For instance, he stated that already “the theory of relativity reminds us of the subjective character of all physical phenomena” ( ATDN , p. 116). Rather, by referring to the subjective character of quantum phenomena he was expressing the epistemological thesis that all observations in physics are in fact context-dependent. There exists, according to Bohr, no view from nowhere in virtue of which quantum objects can be described.

Fourth, although Bohr had spoken about “disturbing the phenomena by observation,” in some of his earliest papers on complementarity, he never had in mind the observer-induced collapse of the wave packet. Later he always talked about the interaction between the object and the measurement apparatus which was taken to be completely objective. Thus, Schrödinger’s Cat did not pose any riddle to Bohr. The cat would be dead or alive long before we open the box to find out. What Bohr claimed was, however, that the state of the object and the state of the instrument are dynamically inseparable during the interaction. Moreover, the atomic object does not posses any state separate from the one it manifests at the end of the interaction because the measuring instrument establishes the necessary conditions under which it makes sense to use the state concept.

It was the same analysis that Bohr applied in answering the challenge of the EPR-paper. Bohr’s reply was that we cannot separate the dynamical and kinematical properties of a joint system of two particles until we actually have made a measurement and thereby set the experimental conditions for the ascription of a certain state value ( CC , p. 80). Bohr’s way of addressing the puzzle was to point out that individual states of a pair of coupled particles cannot be considered in isolation, in the same way as the state of the object and the state of the instrument are dynamically inseparable during measurements. Thus, based on our knowledge of a particular state value of the auxiliary body A, being an atomic object or an instrument, we may then infer the state value of the object B with which A once interacted (Faye 1991, pp. 182–183). It therefore makes sense when Howard (2004, p. 671) holds that Bohr considered the post-measurement joint state of the object and the measuring apparatus to be entangled as in any other quantum interaction involving an entangled pair.

Finally, when Bohr insisted on the use of classical concepts for understanding quantum phenomena, he did not believe, as it is sometimes suggested, that macroscopic objects or the measuring apparatus always have to be described in terms of the dynamical laws of classical physics. The use of the classical concepts is necessary, according to Bohr, because by these we have learned to communicate to others about our physical experience. The classical concepts are merely a refinement of everyday concepts of position and action in space and time. However, the use of the classical concepts is not the same in quantum mechanics as in classical physics. Bohr was well aware of the fact that, on pains of inconsistency, the classical concepts must be given “a suitable quantum-theoretical re-interpretation,” before they could be employed to describe quantum phenomena ( ATDN , p. 8).

The Copenhagen interpretation is not a homogenous view. This insight has begun to emerge among historians and philosophers of science over the last ten to fifteen years. Both James Cushing (1994) and Mara Beller (1999) take for granted the existence of a unitary Copenhagen interpretation in their social and institutional explanation of the once total dominance of the Copenhagen orthodoxy; a view they personally find unconvincing and outdated partly because they read Bohr’s view on quantum mechanics through Heisenberg’s exposition. But historians and philosophers of science have gradually realized that Bohr’s and Heisenberg’s pictures of complementarity on the surface may appear similar but beneath the surface diverge significantly. Don Howard (2004, p. 680) goes as far as concluding that “until Heisenberg coined the term in 1955, there was no unitary Copenhagen interpretation of quantum mechanics.” The term apparently occurs for the first time in Heisenberg (1955). In addition, Howard also argues that it was Heisenberg’s exposition of complementarity, and not Bohr’s, with its emphasis on a privileged role for the observer and observer-induced wave packet collapse that became identical with that interpretation. Says he: “Whatever Heisenberg’s motivation, his invention of a unitary Copenhagen view on interpretation, at the center of which was his own, distinctively subjectivist view of the role of the observer, quickly found an audience” (p. 677). This audience included people like David Bohm, Paul Feyerabend, Norwood Russell Hanson, and Karl Popper who used Heisenberg’s presentation of complementarity as the target for their criticism of the orthodox view. However, it should also be mentioned that in later work, Feyerabend (1968, 1969) was one of the first philosophers who gave a painstaking analysis of complementarity in order to clear up the myth of it being unintelligible. Feyerabend urged philosophers and physicists to go back to Bohr and read him carefully.

Following up on Don Howard’s research, Kristian Camilleri (2006, 2007) points to the fact that complementarity was originally thought by Bohr (in his Como-paper) to exist between the space-time description and the causal description of the stationary states of atoms — and not between different experimental outcomes of the free electron. So the formulation of complementarity was restricted to the concept of stationary states because only there does the system have a well-defined energy state independent of any measurement. This observation deserves general recognition. But when Bohr rather soon thereafter began analysing the double slit experiment in his discussion with Einstein (1930), he had to extend his interpretation to cover the electron in interaction with the measuring apparatus.

Camilleri then shows how Heisenberg’s view of complementarity, in spite of Heisenberg’s own testimony, radically differs from Bohr’s. As Heisenberg understood complementarity between the space-time description and causal description, it holds between the classical description of experimental phenomena and the description of the state of the system in terms of the wave function. A quotation from Heisenberg (1958, p. 50) shows how much he misunderstood Bohr in spite of their previously close working relationship.

Bohr uses the concept of ‘complementarity’ at several places in the interpretation of quantum theory … The space-time description of the atomic events is complementary to their deterministic description. The probability function obeys an equation of motion as did the co-ordinates in Newtonian mechanics; its change in the course of time is completely determined by the quantum mechanical equation; it does not allow a description in space and time but breaks the determined continuity of the probability function by changing our knowledge of the system.

So, where Bohr identified the causal description with the conservation of energy, Heisenberg saw it as the deterministic evolution of Schrödinger’s ψ -function. In other words, Heisenberg, in contrast to Bohr, believed that the wave equation gave a causal, albeit probabilistic description of the free electron in configuration space. It also explains why so many philosophers and physicists have identified the Copenhagen interpretation with the mysterious collapse of the wave packet. The transition from a causal description in terms of the evolution of the ψ -function to a classical space-time description is characterized by the discontinuous change that occurs by the act of measurement. According to Heisenberg, these two modes of description are complementary.

In another study Ravi Gomatam (2007) agrees with Howard’s exposition in arguing that Bohr’s interpretation of complementarity and the textbook Copenhagen interpretation (i.e. wave-particle duality and wave packet collapse) are incompatible. More recently, Henderson (2010) has come to a similar conclusion. He makes a distinction between different versions of Copenhagen interpretations based on statements from some of the main characters. On one side of the spectrum there is Bohr who did not think of quantum measurement in terms of a collapse of the wave function (for a contrasting view see Jens Hebor 2005; and partly Zinkernagel 2016); in the middle we find Heisenberg talking about the collapse as an objective physical process but thinking that this couldn’t be analyzed any further because of its indeterministic nature, and at the opposite side Johann von Neumann and Eugene Wigner argued that the human mind has a direct influence on the reduction of the wave packet. Unfortunately, von Neumann’s dualistic view has become part of the Copenhagen methodology by people opposing this interpretation.

Some physicists and philosophers of science see the measurement problem as a real puzzle with respect to the Copenhagen interpretation. On the one hand, we have that the wave function evolved deterministically according to Schrödinger’s equation as a linear superposition of different states; and the other hand an actual measurement always detect one definite state. To be or not to be in a superposition; that’s the question.

Apparently, we are living in a quantum world since everything is constituted by atomic and subatomic particles. Hence classical physics seems merely to be a useful approximation to a world which is quantum mechanical on all scales. Such a view, which many modern physicists support, can be called quantum fundamentalism (Zinkernagel 2015, 2016). It can be defined as a position containing two components: (1) everything in the Universe is fundamentally of quantum nature (the ontological component); and (2) everything in the Universe is ultimately describable in quantum mechanical terms (the epistemological component). Thus, we may define quantum fundamentalism to be the position holding that everything in the world is essentially quantized and that the quantum theory gives us a literal description of this nature. The basic assumption behind quantum fundamentalism is that the structure of the formalism, in this case the wave function, corresponds to how the world is structured. For instance, according to the wave function description every quantum system may be in a superposition of different states because a combination of state vectors is also a state vector. Now, assuming that both the quantum object and the measuring apparatus are quantum systems that each can be described by a wave function, it follows that their entangled state would likewise be represented by a state vector. Then the challenge is, of course, how we can explain why the pointer of a measuring instrument enters a definite (and not a superposition) position, as experience tells us, whenever the apparatus interacts with the object. In a nutshell this is the measurement problem.

The Copenhagen interpretation is often taken to subscribe to a solution to the measurement problem that has been offered in terms of John von Neumann’s projection postulate. In 1932 [1996], von Neumann suggested that the entangled state of the object and the instrument collapses to a determinate state whenever a measurement takes place. This measurement process (a type 1-process as he called it) could not be described by quantum mechanics; quantum mechanics can only described type-2 processes (i.e., the development of a quantum system in terms of Schrödinger’s equation). In his discussion of the measurement problem, von Neumann then distinguished between (i) the system actually observed; (ii) the measuring instrument; and (iii) the actual observer. He argues that during a measurement the actual observer gets a subjective perception of what is going on that has a non-physical nature, which distinguishes it from the observed object and the measuring instrument. However, he holds on to psycho-physical parallelism as a scientific principle, which he interprets such that there exists a physical correlate to any extra-physical process of the subjective experience. So in every case where we have a subjective perception we must divide the world into the observed system and the observer. But where the division takes place is partly arbitrary. According von Neumann, it is contextual whether the dividing line is drawn between the description of the observed object (i) and the measuring instrument together with the observer (ii) + (iii), or it is drawn between the description of the observed object together with the measuring instrument, i.e., (i) + (ii), and the observer (iii). In other words, von Neumann argues that the observer can never be included in a type 2-process description, but the measuring instrument may sometime be part of a type 2-process, although it gives the same result with respect to the observed object (i). An important consequence of von Neumann’s solution to the measurement problem is that a type 1-process takes place only in the presence of the observer’s consciousness. Furthermore, even when von Neumann considers the situation in which the descriptions of (i) and (ii) are combined, he talks about the interaction between the physical system (i) + (ii) and an abstract ego (iii) (Neumann 1932 [1996], Ch VI). Therefore, the mind seems to play an active role in forming a type 1-process, which would be incompatible with psycho-physical parallelism.

Indeed, within philosophy of mind one cannot consistently maintain both psycho-physical parallelism and the existence of an interaction between the brain and the mind. So it is no wonder that Eugene Wigner (1967) followed up on the suggestion of the mind’s interaction by proposing that what causes a collapse of the wave function is the mind of the observer. But Wigner never explained how it was possible for something mental to produce a material effect like the collapse of a quantum system. The measuring problem led to the famous paradox of Schrödinger’s cat and later to the one of Wigner’s friend. Although von Neumann’s and Wigner’s positions are usually associated with the Copenhagen Interpretation, such views were definitely not Bohr’s as we shall see in a moment.

Quantum fundamentalists must indeed be ready to explain why the macroscopic world appears classical. An alternative to von Neumann’s projection postulate is the claim that the formalism should be read literally and that measurements (classical outcomes) do not describe the world as it really is. But there are ontological cost, which is significant to some. In one interpretation the world divides into as many worlds as there are possible measurement outcomes each time a system is observed or interacts with another system. Other fundamentalists had hoped that the decoherence program might come up with an appropriate explanation. The decoherence theory sees entanglement to exist not only between object and the measurement but also as something which includes the environment. If Bohr had known the idea of decoherence, he would probably have had no objection to it, as several authors have pointed to decoherence as a natural dynamical extension of his view that measurements is an irreversible amplification process (Schlosshauer and Camilleri 2015, 2017; Bächtold 2017, Tanona 2017; and Dieks 2017). However, it is generally agreed that decoherence does not solve the measurement problem (Bacciagaluppi 2016; Zinkernagel 2011). This might seem as if von Neumann’s projection postulate has to be reintroduced as a dynamical factor to explain why one and only one measurement result appears. However, as Dieks (2017) argues, Bohr’s interpretation could be understood as a non-collapse interpretation, since “the superposition does not have an empirical meaning independently of its interpretation via classically described experiments, so no replacement by another mathematical state is needed. We just have to interpret the formulas correctly.” In spite of that there is no general agreement to what extent Bohr opposed quantum fundamentalism.

Time and again Bohr emphasized that the epistemological distinction between the instrument and the object is necessary because this is the only way one can functionally make sense of a measurement. The epistemic purpose of a measuring instrument is to yield information about an object separated from the instrument itself. It is also generally agreed that Bohr didn’t treat the classical world of the measuring instrument as epistemically separated from quantum object along the line of a microscopic and macroscopic division. Where the line is drawn is a matter of convenience. Thus, Bohr sometimes included parts of the measuring instrument into the system to which the quantum mechanical description should be applied. In one place Bohr gave the following characteristics of the situation:

In the system to which the quantum mechanical formalism is applied, it is of course possible to include any intermediate auxiliary agency employed in the measuring processes. Since, however, all those properties of such agencies which, according to the aim of the measurement, have to be compared with corresponding properties of the object, must be described on classical lines, their quantum mechanical treatment will for this purpose be essentially equivalent with a classical description. The question of eventually including such agencies within the system under investigation is thus purely a matter of practical convenience, just as in classical physical measurements; and such displacements of the section between object and measuring instruments can therefore never involve any arbitrariness in the description of a phenomenon and its quantum mechanical treatment. (Bohr, CC , p. 104)

This is an important passage because it indicates that Bohr believed that quantum mechanics has a universal applicability in the sense that it can be used to describe macroscopic objects as well. It also tell us that whether or not parts of the macroscopic measuring apparatus are treated as a part of the quantum object depends on pragmatic or contextual conditions, but their quantum mechanical treatment should result in a description that corresponds to a similar classical description. Based on quotes like the one above, Don Howard (1994) concludes that Bohr was not only an ontological quantum fundamentalist but in fact also a sort of an epistemological one. He believes that one can make Bohr’s requirement that measuring apparatus and the experimental results have to be described in ordinary language supplemented with the terminology of classical physics consistent with ontological quantum fundamentalism. According to him, Bohr never considered the measuring instrument as a classical object. Moreover, he thinks that this implies that Bohr had to understand the use of classical concepts differently from what scholars usually think. He reinterprets Bohr in terms of quantum states called “mixtures”. Howard believes that with respect to an experimental context in which an instrument interacts with an object, Bohr didn’t understand them as being in an entangled state but being separated in a mixture state. The consequence would be that the instrument and the object exist in a definite quantum state since such a state could be represented as a product of the wave function for the instrument and for the object.

But, as Maximilian Schlosshauer and Kristian Camilleri (2008 (Other Internet Resources), 2011) have pointed out, this does not solve the measurement problem. Howard does not explain under which circumstances one can move from a quantum system-cum-measuring apparatus being in a non-separable state to a mixture of separated states. Therefore one cannot be sure that the measuring apparatus is in a definite state and its pointer in a definite place. Some philosophers seem to argue that Bohr was an ontological but not an epistemological quantum fundamentalist. For instance, “Bohr believed in the fundamental and universal nature of quantum mechanics, and saw the classical description of the apparatus as a purely epistemological move, which expressed the fact that a given quantum system is being used as a measuring device” (Landsman 2007); and in a similar vein: “One is left with the impression from Bohr’s writings that the quantum-classical divide is a necessary part of the epistemological structure of quantum mechanics” (Schlosshauer and Camilleri 2008 (Other Internet Resources), 2015). So Klaas Landsman (2006, 2007) accepts Howard’s suggestion that Bohr is an ontological quantum fundamentalist but he rejects that Bohr should be considered an epistemological quantum fundamentalist. Landsman argues that Bohr held that the measuring instrument should be described in classical terms since the results of any measurement like in classical physics would always have a definite value. However, Landsman agrees that Bohr understood all objects as essentially quantum mechanical objects.

However, one could argue that Howard and Landsman miss the epistemic nature of Bohr’s view on ontological issues. Apparently, Bohr might say that quantum mechanics is correctly understood a theory of measurement, a theory about how the quantum world appears to us, and not how this world is in itself. Bohr mentioned more than once that physics is not about finding the essence of nature but about describing the phenomena in an unambiguous way. In the foreground of Bohr’s thinking was the (1) the need of classical concepts for the description of measuring results; (2) non-separability due to the entanglement of the system and the measuring instrument; (3) the contextual nature of the measurements of complementary properties; and (4) the symbolic character of the quantum formalism. One has to take all four components into consideration if one wants to understand Bohr’s solution to the classical-quantum problem.

According to this understanding of Bohr as an empirical realist but a transcendental idealist, we are in quantum mechanics confronted with the “impossibility of any sharp separation between the behavior of atomic objects and the interaction with the measuring instruments which serve to define the very conditions under which the phenomena appear” ( APHK , p. 210). This is definitely a non-classical feature which is described by quantum mechanics alone. In his response to the EPR-paper, Bohr strongly rejected that this form of interaction could be regarded as a mechanical influence. The influence was on the conditions of description, i.e. the experimental conditions under which it makes sense to apply classical concepts. But during a measurement we need to separate the system from the measuring instrument and the environment for pragmatic reasons. The pragmatic reasons seem to be reasonably clear. The outcomes of whatever experiment always yield a definite value, so the entanglement of object and the measurement instrument described by the quantum formalism only lasts epistemically until the interaction between object and instrument stops. The quantum formalism can predict the statistical outcome of these interactions but it cannot say anything about the trajectory of objects.

Bohr’s firmness about the use of classical concepts for the descriptions of measurement can be seen as his response to the measurement problem. This problem arises from the fact that quantum mechanics itself cannot account for why experiments on objects in a state of superposition always produce a definite outcome. Hence if one does not argue for spontaneous collapse of the wave function, hidden variables, or many worlds, one needs to supplement quantum mechanics with a classical description of measuring instruments in terms of clocks and rods. Henrik Zinkernagel (2015, 2016) may seem to get close to Bohr’s view when he argues that Bohr not so much solved the measuring problem as he dissolved it. According to his interpretation, Bohr believed in a quantum world, but only relative to a particular classical description and a certain classical world. The distinction between classical and quantum (both ontic and epistemic) is contextual. He thinks that the measurement problem is ultimately a consequence of ontological quantum fundamentalism (that everything is quantum). Because if everything is quantum – and correctly described by quantum formalism (what else would it mean to call everything quantum?) – then a measurement ends up in a superposition whether we describe the apparatus classical or not. One could say with Zinkernagel that Bohr believed all objects can be treated as quantum objects, but they cannot all be treated as quantum objects at the same time. Borrowing a conception from the two Russian physicists, Landau and Lifshitz, Zinkernagel claims that only some parts of the measuring device are entangled with the object in question, but those parts which are not entangled exists as a classical object. Depending on the context, objects cannot be treated as quantum objects in those situations in which they acts as measuring apparatuses. In these situations the classical treatment of the measuring device provides us with a frame of reference of space and time with respect to which an atomic object has a position, and, mutatis mutandis, with respect to which it has energy and momentum. Such a frame of reference is necessary for our ability to define and measure a particular property. In Bohr’s own words: “in each case [of measurement] some ultimate measuring instruments, like the scales and clocks which determine the frame of space-time coordination on which, in the last resort, even the definitions of momentum and energy quantities rest, must always be described entirely on classical lines, and consequently kept outside the system subject to quantum mechanical treatment” ( CC , p. 104). What characterizes a frame of reference is that it establishes the conditions for the ascription of a well-defined position or a well-defined momentum, and treated classically measuring instruments act exactly as frames of reference. The implication is that Bohr did not exclude the application of quantum theory to any system. Every system can in principle be treated quantum mechanically, but since we always need a frame of reference to describe experimental outcomes, not all systems can be treated quantum mechanically at once.

In this debate Dorato (2017) stresses the fact that by making explicit reference to Einstein’s presentation of his special theory of relativity, Bohr regarded quantum mechanics as a theory of principle. This explains both Bohr’s epistemic reliance on the domain of classical physics and his ban of any attempt to construct classical objects from quantum objects. Despite this position Dorato argues that in order to justify his entity realism and anti-instrumentalist interpretations, Bohr also needed to postulate something ontologically distinct from the realm of quantum mechanics, a claim that creates the well-known problem of defining in a non-ambiguous and exact way the cut between the classical and the quantum realm. By following Zinkernagel, he claims that this problem is somewhat softened by Bohr’s contextualist theory of measurement. However, Bohr’s holism, according to which the measuring device and quantum object are in state of entanglement, is in objective tension with Bohr’s thesis of an ontological distinction, especially in virtue of the fact that by referring to the interaction between the quantum and the classical system as an irreversible physical process, Bohr seems to need a constructive approach to quantum mechanics that he wants to avoid.

Nonetheless, the question is to what extent Bohr really believed that the classical world is not only epistemically but also ontologically different from the quantum world? If he did not make an ontological distinction, there would be no contradiction between his epistemic view that the outcome of measurement needs to be described classically but that the apparatus ontologically is just as much a quantum object as the object under investigation. So when Bohr regarded quantum mechanics as a rational generalization of classical physics, he always thought of it as a way to secure the epistemic validity of quantum mechanics and not a way to save a classical ontology. Directly addressing Zinkernagel’s analysis, Dieks (2017) strongly argues that there can be little doubt that Bohr believed that quantum mechanics is universal in the sense that Heisenberg’s indeterminacy relation applies to both micro- and macroscopic systems due to the quantum of action. Classical mechanics is a mathematical approximation. Moreover, Bohr believed for epistemic reasons that we had to use classical language because this language is a refinement of our everyday language, which is adapted to describe our sensory experience and therefore the only language that can endow the quantum formalism with an empirical content. Hence, according to Dieks, Bohr assumed that it is only an epistemic necessity to describe “ some systems classically in order to have a pragmatic starting point for the treatment of other systems.” Bohr’s demand of using classical concepts for epistemic reasons has no implications for his view that macroscopic objects are quantum objects. Measuring devices are not classical objects even though we need classical concepts to describe our general physical experiences and the outcome of quantum experiments. So Dieks concludes that the interaction between the measuring device and the quantum object determines, in the classical textbook examples, whether position or momentum talk can be carried over to the quantum object that is measured. The measuring device itself, if macroscopic and under ordinary circumstances (so that it really is a measuring device that can be used by us) allows both position and momentum talk in its own description. The measurement interaction determines which correlations are forged with the micro-world.

Everyone agrees that decoherence does not provide physicists with an explanation of why a measurement selects one particular value rather than another given all the statistically possible values represented by the wave function. But the question is still open regarding in what way Bohr thought that any measuring process rests on a dynamical interaction between the quantum system and the measuring apparatus, in spite of the fact that a descriptive separation between them had to be made if the value offered by the measuring apparatus has to be a recording of the quantum system. The closest Bohr came to such a dynamical interaction was when he maintained that the measuring process leaves an irreversible mark on the instrument. In spite of such remarks, Bohr never discussed his interpretation in modern terms of “entanglement” or “decoherence”.

While Schrödinger coined the term “entanglement”, Howard (2021) argues that the word refers to an unavoidable feature of quantum mechanics, which most physicists recognized from the very start. Einstein for one already realized this in connection his concept of light quanta with respect to the photoelectric effect. Bohr’s formulation of entanglement was the uncontrollable interaction between the object and the measuring apparatus. For instance, when Bohr presented his view on complementarity in Como 1927 he wrote:

the quantum postulate implies that any observation of atomic phenomena will involve an interaction with the agency of observation not to be neglected. Accordingly, an independent reality in the ordinary physical sense can neither be ascribed to the phenomena nor to the agencies of observation. ( ATDN , p. 54)

As Howard (2021) points out, we know that the interaction Bohr here talked about, is not the one Heisenberg had in mind when he presented his electron microscope in which light used to detect the electron disturbs the electron. Such an explanation was exactly the suggestion that Bohr, in his discussion with Heisenberg, had turned against before the latter published his classical paper on the indeterminacy relation.

However, Bohr’s formulation has often been misread as giving support to a classical form of disturbance, and the expression “uncontrollable interaction” may have misled Einstein and other physicists to believe that quantum mechanics was incomplete, because to be considered as an disturbance such an interaction may be thought to change a pre-existing value. The first time Bohr publicly distanced himself from such a disturbance interpretation of interaction between the object and the instrument was in his reply to the EPR-paper. In this case, the entanglement under consideration is no longer the one between an object and the measuring instrument but between two subsystems A and B. Nonetheless, presenting an experiment in which an object first passes either a fixed or a movable single slit diaphragm, and then another diaphragm with a double slit, Bohr showed with his example how the outcome of particular measurement on subsystem A influences the outcome of measurement on B. (If the first diaphragm is fixed in order to measure the object’s position, one observes on the screen behind the second diaphragm an interference pattern, but if first diaphragm is movable in order to determine through which one of the two slits the object travelled, the interference pattern disappears).

Of course there is in a case like that just considered no question of a mechanical disturbance of the system under investigation during the last critical state of the measuring procedure. But even at this stage there is essentially the question of an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system . ( CC , p. 80)

Here Bohr explicitly denied that the uncontrollable interaction involves any form of disturbance, which is otherwise characteristic of well-separated objects in classical states. The uncontrollable interaction he had in mind was what in modern term is a state in which the system and the measuring instrument is entangled. The entanglement between the object and the measuring apparatus prevents the object as well as the detector from having a well-defined, independent state. Hence, the entanglement prompted Bohr to think that the use of classical concepts depended on the experimental context and that was therefore complementary.

Today most philosophers agree that ontologically, Bohr was a quantum fundamentalist, holding that the quantum mechanical description applies to microscopic as well as macroscopic objects. However, he also insisted that measuring results should be described in classical terms due to the epistemic separation we have to make between the entangled object and the measuring instrument. This is how we can communicate our observation. The question then arises: how is it that the world appears in a classical manner?

In 1970 quantum mechanics was enriched with a new notion when Hans Dieter Zeh (1970) introduced ‘decoherence’. Since then, many physicists have taken part in developing this notion, and today the processes behind decoherence plays a huge role; for instance, in the construction of quantum computers. The view behind decoherence is that quantum mechanics rules all the way up and down but that any object is always interacting with a very large number of environmental degrees of freedom such that the interference terms corresponding to macroscopically distinct states disappear. In other words, we do not see Schrödinger’s cat in a superposition in which it is both dead and alive.

Camilleri and Schlosshauer (2015) and Schlosshauer and Camilleri (2017) have argued that Bohr’s philosophy and the insights brought about by decoherence theory may not just coexist peacefully. Decoherence may also be seen as supplying the dynamical justification for the distinction that Bohr held to be epistemically necessary, between the “classical system” serving as the measuring apparatus and the “quantum object.” However, Howard (2021) goes a step further as he argues that Bohr believed as part of his view of complementarity that Schrödinger’s dynamics of pure joint states of the object-instrument-environment interaction is statistically and observationally equivalent to the context-dependent mixture over eigenstates picked out by measurements. Howard therefore argues: (1) complementarity and decoherence assume that quantum is ontologically fundamental; (2) classicality emerges from quantum behavior; and (3) decoherence confirms Bohr’s view that measurement is a practical irreversible process where information dissipates through the observed system due to the fact that decoherence produces a single macroscopic mark. His conclusion is that the “environmentally-induced decoherence (which is not really decoherence, but only a semblance thereof) was, all along, the real point toward which Bohr was gesturing with the doctrines of complementarity” (p. 171). Hence, Howard concludes that decoherence is not only in agreement with complementarity, but complementarity is a consequence of the fact that quantum mechanics with the Born rule is a statistical theory and so is the decoherence dynamics, and therefore we cannot expect that decoherence should be able to explain a specific determinate outcome of a measurement. However, he also concludes that the context-dependent mixture he reads into Bohr’s view provides us with “the sought-for simulacrum of classicality”. Whether Bohr would support the last remark that the classical world is an epistemic illusion is certainly debatable.

Howard’s reading seems to rest on an understanding of Schrödinger’s wave function as having a representational role of referring to dynamical quantum states. However, Bohr never seems to have spoken about quantum states other than in connection with the stationary states of an atom. From the very beginning, Bohr focused on the transition probabilities of Heisenberg’s matrix mechanics more than on Schrödinger’s wave mechanics, a view that did not change after the interpretation of the wave function in terms of the Born rule. Consider the following statements by Bohr:

According to the two alternative procedures, quantum mechanical calculations may be performed either by representing the variables with matrices with elements referring to the individual transitions between two states of the system or by making use of the so called wave equation, the solutions of which refer to these states and allow us to derive probabilities for the transitions between them. The entire formalism is to be considered as a tool for deriving predictions of definite or statistical character, as regards information obtainable under experimental conditions described in classical terms and specified by means of parameters entering into the algebraic or differential equations of which the matrices or the wave-functions, respectively, are solutions. These symbols themselves, as is indicated already by the use of imaginary numbers, are not susceptible to pictorial interpretation; and even derived real functions like densities and currents are only to be regarded as expressing the probabilities for the occurrence of individual events observable under well defined experimental conditions. ( CC , pp. 143–144)

Here Bohr equates matrices and wave functions because these two sets of mathematical symbols both express probabilities in quantum mechanics in the form of predictions of measurement outcomes. Following David Wallace (2016) in distinguishing between representational and probabilistic interpretations of quantum states, Jeffery Bub (2017) argues that Bohr interpreted them probabilistically, which the above quotation seems to support.

Bohr’s primary insight was to see that quantum mechanics is quite unlike any theory we have dealt with before in the history of physics, and so explanation in such a post-classical theory can’t be the sort of representational explanation we are familiar with in a theory that is commutative or Boolean at the fundamental level.

As Bub (2016) explains, “quantum probabilities can’t be understood in the classical or Boolean sense as quantifying ignorance about pre-measurement value of observable” (p. 223). Thus, when Heisenberg constructed his matrix mechanics, he realized that kinematical and dynamical quantities in quantum mechanics are non-communitive. This implies, according to Bub, that these properties must be described by a family of “intertwined” Boolean algebras, each member corresponding to a set of commuting observables, where intertwinedness excludes the possibility of constructing a more inclusive Boolean algebra. Although Bohr did not speak about complementarity in terms of Boolean algebras, Bub suggests that this is exactly what Bohr had in mind when he insisted that we needed to use classical terms in the description of measurement outcomes. On the macroscopic level, depending on the experimental context, the various outcomes belong to incompatible Boolean frames; on the microscopic level randomness and entanglement rules, something which quantum mechanics explains in terms of probabilistic constraints on events depicted through the geometry of Hilbert space.

This information-theoretic interpretation of complementarity, according to Bub (2022), abstains from introducing decoherence to explain quantum probabilities, which are ‘perfectly new and sui generis aspects of physical reality’ and ‘uniquely given from the start’, as von Neumann had pointed out. In addition to this, Bub (2017) argues that “The collapse, as a conditionalization of the quantum state, is something you put in by hand after observing the actual outcome. The physics doesn’t give it to you.” This might well be Bohr’s view. No matter what, the divergences between the various analyses show that there is no consensus about how Bohr’s view on complementarity and measurement plays out with respect to one’s favorite quantum foundation.

After the 1950s a number of alternative interpretations to Bohr’s complementarity were articulated and they all found their proponents among physicists and philosophers of science. The Copenhagen interpretation started to lose ground to other interpretations such as Bohm’s interpretation, the many worlds interpretation, the modal interpretation and the decoherence interpretation, which have been more in vogue the last couple of decades. But parallel with the growing awareness of the essential differences between Bohr’s and Heisenberg’s understanding of quantum mechanics several philosophers of science have revitalised Bohr’s view on complementarity. Around the millennium a new recognition of the Copenhagen interpretation has emerged.

Rob Clifton and Hans Halvorson (1999, 2002) argue that Bohm’s interpretation of quantum mechanics can be seen as a special case of Bohr’s complementarity interpretation if it is assumed that all measurements ultimately reduce to positions measurement. Originally Jeffrey Bub and Clifton (1996) were able to demonstrate (given some idealized conditions) that Bohr’s complementarity and Bohm’s mechanics fall under their uniqueness theorem for no-collapse interpretations. Clifton and Halvorson improve this result by showing that Bohr’s idea of position and momentum complementarity can be expressed in terms of inequivalent representations in the C*-algebraic formalism of quantum mechanics. It turns out that either position or momentum are dynamically significant, but it is not permissible to assume that position and momentum are both dynamically significant in any single context. From these assumptions they conclude that Bohm’s hidden variables are none other than the “value states” that the complementarity interpretation postulates if position measurement were always dynamically significant, but this metaphysical restriction is not, as their results indicate, demanded by the physics. Rather, Clifton and Halvorson (1999) and Halvorson (2004) believe that complementarity may give us a realist interpretation of quantum field theory.

Philosophers have also started to explore the idea of decoherence in relation to Bohr’s view about “the inseparability of the behavior of the object and the interaction with the measuring instrument” or “the uncontrollable interaction between the atomic system and measurement apparatus.” (Schlosshauer and Camilleri 2011, 2017; Camilleri and Schlosshauer 2015; Bächtold 2017; and Tanona 2017). Although Bohr assumed that the measuring apparatus is altogether a quantum mechanical system, he nevertheless believed that the instrument could be approximately described by classical theory . Among the scholars just mentioned there is a general agreement that the notion of decoherent is coherent with Bohr’s view about the quantum-classical division and adds a dynamical explanation of quantum-to-classical transition which Bohr’s own exposition lacked. Also attempts to clear up the structural relationship between Bohr’s view and Hugh Everett’s “relative state”-interpretation have been carried out; a relationship which at some points is much closer than usually thought (Bacciagaluppi 2017).

Another insight into Bohr’s view of complementarity is due to Michael Dickson (2001, 2002). By using the contemporary theory of reference frames in quantum theory, he proves that Bohr’s response to the EPR thought-experiment was in fact the correct one. Moreover, he also maintains that Bohr’s discussions of spin, a property much less frame dependent than position and momentum, were very different from his discussions of the latter, and based on these differences he offers a Bohrian account of Bell’s theorem and its significance.

A re-assessment of Bohr’s philosophy of quantum mechanics is made by Whitaker (2004) on the basis of Clifton and Halvorson’s and Dickson’s works and in the light of quantum information theory. Besides these attempts to apply Bohr’s notion of complementarity to the contemporary discussions of the interpretation of quantum mechanics and quantum field theory there is an ongoing attempt to understand Bohr’s idea of symbolic representation (Tanona, 2004a, 2004b) and his notion of complementarity with respect to trends in post-modern philosophy and general epistemology such as poststructuralism, deconstructivism, feminism and cultural studies (Honner 1994; Plotnitsky 1994; Barad 2007; and Katsumori 2011). Faye and Jaksland (2023b) offers a critical discussion of Barad’s interpretation of Bohr.

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• Kober, M., 2009, “ The Copenhagen Interpretation of Quantum Theory and the Measurement Problem ,” at arXiv.org.
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Otto Stern (1888–1969): The founding father of experimental atomic physics

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Friedrich,  Bretislav Molecular Physics, Fritz Haber Institute, Max Planck Society;

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Toennis, J. P., Schmidt-Böcking, H., Friedrich, B., & Lower, J. C. A. (2011). Otto Stern (1888–1969): The founding father of experimental atomic physics. Annalen der Physik, 523 (12), 1045-1070. doi:10.1002/andp.201100228.

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• DOI: 10.1007/978-3-030-63963-1_5
• Corpus ID: 237972770

Otto Stern’s Molecular Beam Method and Its Impact on Quantum Physics

• B. Friedrich , H. Schmidt-Böcking
• Published in Molecular Beams in Physics… 2021
• Molecular Beams in Physics and Chemistry

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Zig-zag dynamics in a stern–gerlach spin measurement, stern-gerlach interferometry with the atom chip, die lücke als fund: über eine fehlstelle zur familiengeschichte im nachlass von walther gerlach (1889–1979) **, temperature-dependent dielectric function of intrinsic silicon: analytic models and atom-surface potentials, a century ago the stern–gerlach experiment ruled unequivocally in favor of quantum mechanics, 140 references, the stern-gerlach experiment revisited, visualisation of quantum evolution in the stern-gerlach and rabi experiments., otto stern (1888–1969): the founding father of experimental atomic physics, coherent stern–gerlach momentum splitting on an atom chip, consistent quantum measurements, the quantum theory of the electron, reduction of the atomic wavefunction in the stern-gerlach magnetic field, much polyphony but little harmony: otto sackur’s groping for a quantum theory of gases, the stern-gerlach experiment and the effects of spin relaxation., multiple perspectives on the stern-gerlach experiment, related papers.

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Prof. dr. isidor isaac rabi > research profile.

by Luisa Bonolis Isidor Isaac Rabi Nobel Prize in Physics 1944 "for his resonance method for recording the magnetic properties of atomic nuclei".

As Norman Ramsey, one of Isidor Rabi's biographers emphasised, "Some scientists make their greatest contribution through their own personal research, while others are best remembered for their general wisdom and their influence on others. A few, including Rabi, excel in both respects." It would actually be reductive to talk of Rabi's important discoveries, which led to his Nobel Prize in 1944, without mentioning how his influence extended far beyond his own laboratory and how, under his visionary leadership as a statesman of science, many successful ventures in national and international cooperation in science were realised. In particular, he was one of the founders of the Brookhaven National Laboratory and a main promoter of the CERN laboratory. His great reputation and his contacts, with the leading physicists as well as with the government leaders of the United Nations, turned into valuable tools when he became a spokesman for the peaceful use of nuclear energy.

Becoming an adept in quantum theory Isidor Isaac Rabi was born in Rymanow, Austria-Hungary, in 1898, at the very end of the 19th century, when X-rays, radioactivity and the electron were discovered. The following year his parents moved to New York City where he attended public school, but gaining much of his education and interest in science through books borrowed from the public library. In 1916, after graduating from high school, Rabi entered Cornell University with a scholarship, starting in electrical engineering, but graduating in the field of chemistry. After three years far from university, he returned first to Cornell, to do graduate work in chemistry, moving a year later to Columbia University and turning to physics. In 1923, when Rabi was beginning his physics studies, he discovered that his real interest was the quantum theory. However, no physics professor at Columbia was really conversant with such novelties coming from Europe and he had to choose a dissertation topic that involved measuring the magnetic susceptibility of a series of crystalline salts. In the meantime he organised a study group of fellow students to grapple with quantum mechanics. In July 1927, Rabi submitted his doctoral thesis to the journal Physical Review, and the next day he married Helen Newmark. Soon after, like many other US young physicists, he went on a travelling fellowship to Europe, in order to have a closer view of the pioneers of the new quantum mechanics.

A European Tour through the Centres of Quantum Mechanics During the first months Rabi visited Erwin Schrödinger in Zurich, Arnold Sommerfeld in Munich, and Niels Bohr in Copenhagen. The latter arranged for Rabi to stay in Hamburg, with Wolfgang Pauli, who at the time was a collaborator of Otto Stern, one of the founding fathers of experimental atomic physics not including spectroscopy. In late October, Rabi arrived there with Yoshio Nishina, who was visiting Europe from Japan. Rabi well knew the Stern-Gerlach experiment of 1922, which had turned out to be one of the milestones on the path to modern quantum physics. In setting up this experiment, Stern was guided by Sommerfeld's extension of the Bohr theory of the atom – an extension independently put forward by Peter Debye – in which, in addition to the usual quantum numbers for the size and shape of orbits, a quantisation of the spatial orientation of the "Keplerian" electron orbits around the nucleus, was proposed, a proposal referred to as space quantisation. Because of the orbital motion of a single electron, an atom can possess a magnetic moment that determines its interaction with external electric and magnetic fields. Spatial quantisation allowed only selected discrete orientations of each atomic magnet relative to the direction of an externally applied magnetic field. In the Stern-Gerlach experiment a collimated beam of silver atoms, all with the same magnetic moment, flowing from a tiny hole of a heated furnace and moving with thermal velocities, passed through a strong non-uniform magnetic field. On its path between the furnace and the detector, the magnetic field will exert a torque on the magnetic dipole, which will thus precess about the direction of the magnetic field. The non-uniform field will also exert on the magnetic moment a transverse force, whose magnitude and direction depend on the orientation of the atom's magnetic moment relative to the direction of the externally applied magnetic field. The component of the magnetic moment parallel to the field direction will not be affected. The classical picture includes no restriction on the angle at which the atomic magnet can precess about the magnetic field. The expectation is that, due to thermal effects in the oven, the magnetic dipole moments of the atoms will be randomly oriented in space with respect to the direction of the field. The directions of motion of the atoms in the initial beam would be displaced by random amounts perpendicular to the direction of motion of the initial beam. A continuous gradation of deflections should thus occur, and the transmitted beam would merely spread out like a fan. In actual fact, Stern and Gerlach found that on the cold glass detector plate the parent beam split into two distinct parts -with no trace of silver atoms in the central region, where one would have expected the undeflected atoms- implying that, in the case of the silver atoms, only two distinct orientations are allowed with respect to the direction of the magnetic field. Stern and Gerlach thus considered their result a decisive refutation of the classical theory, disproving the classical Larmor theory, which was based on continuous values for the direction of the magnetic moments. But at the same time, they mistakenly considered the phenomenon a confirmation of the old quantum theory, according to which the magnetic moment of the silver atom was due to the electrons' orbital angular momentum. Unknowingly, they had actually been the first to observe the quantisation of magnetic moment associated with electron spin, because their silver atoms were actually in the ground state, with total orbital magnetic moment equal to zero, so that the magnetic dipole moment of the atom was entirely due to the spin of the electron, a new quantum number that would be introduced in 1925 by George Uhlenbeck and Samuel Goudsmit. The Stern-Gerlach experiment, an early triumph of the molecular beam method, offering other-than-spectroscopic evidence that quantum objects exhibit behaviour incompatible with classical physics, had stunned and intrigued Rabi as a student, when he was still sceptical about the quantum theory. He became convinced that the system of ideas underlying the Bohr atom and the attempts to extend these ideas to other atomic phenomena were well founded and began to study and discuss with his friends all the papers that would be gradually incorporated into the formal structure of the new quantum mechanics. While working with Nishina and Pauli on theoretical work, he spent some time in Stern's laboratory and carried out successfully what became his first molecular-beam experiment. The magnetic field configuration he designed to deflect the beam particles became known as the Rabi field. Rabi's work in Stern's laboratory was decisive in turning his interest toward molecular beam research. After Hamburg, Rabi went to Leipzig to work with Werner Heisenberg, but in the meantime Pauli left Hamburg for a chair in Zurich and in March 1929 Rabi and Robert Oppenheimer, whom he had met for the first time in Leipzig, followed him to Zurich. Once again it was a wonderful occasion of becoming acquainted with some of the finest minds in physics, but his stay in Zurich ended when, at the end of March, Rabi received a cable from Columbia University, offering him a lectureship at the physics department. They were searching for a theoretical physicist, who could teach the new quantum mechanics and Heisenberg himself, during a visit at Columbia, had strongly recommended Rabi for such a position. He accepted promptly, and on August 1, 1929, he left Europe with his young wife. His scientific apprenticeship had ended, he had developed a new awareness and knowledge of physics at the very sources of the new quantum mechanics.

Molecular Beams to Probe the Nucleus Rabi devoted his first year at Columbia as lecturer exclusively to the strenuous effort of teaching the most advanced courses in the department. Thus began his pervasive influence on American physics. During the following two years he did theoretical research in solid-state physics, but his thoughts were very often directed to molecular beams. In 1931, Harold Urey, Rabi's Columbia colleague, was attempting to determine the nuclear spin of sodium by an analysis of its spectrum, with inconclusive results. At the time, his longstanding involvement in isotope research was inspiring him to search for deuterium, the hydrogen-2 isotope, whose existence he actually announced in The Physical Review on New Year's Day, 1932. For this discovery Urey was then awarded the Nobel Prize in Chemistry 1934. Just seven weeks later, James Chadwick announced the ''possible existence of a neutron,'' a fundamental discovery which officially opened the nuclear era. However, in 1931 the neutron was not yet there and the atomic nucleus was still a terra incognita , an unexplored territory soon to become the domain of Rabi's scientific adventure. Rabi saw that the molecular beam technique could be used to tackle the challenge offered by the uncertainty related to the nuclear spin of sodium. It could provide access to fundamental questions related to both the quantum world and the nuclear realm. Rabi wanted to measure the magnetic moment of a nucleus in the way that Stern had measured the magnetic moment of a silver atom. However, many refinements were required to transform the basic Stern-Gerlach experiment into a technique that could be used for quantitative measurements. In principle, nuclear magnetic properties could be determined through the analysis of atomic spectra, but owing to the minute size of the nuclear moments – three orders of magnitude smaller than their electronic counterparts – experimental techniques were strained to the limit and it was quite difficult to get this kind of information via spectroscopy. Application of an experiment of the Stern-Gerlach type to the measurement of nuclear magnetic properties would provide an independent check on the difficult spectroscopic methods, and at the same time provide access to nuclear data that were otherwise unavailable. With Gregory Breit, his colleague from New York University, Rabi had set up a joint seminar to explore and discuss atomic-nuclear phenomena. In 1931, they developed a formula that showed the variation of the magnetic moment of an atom for the different Zeeman levels of hyperfine structure under the influence of an external magnetic field. The beam method could thus be used to investigate the nuclear magnetic properties of atoms. With Victor Cohen, his first graduate student, Rabi began his pioneering experimental work on the precise measurement of nuclear properties, which brought him to the forefront of nuclear physics during the following decade. By varying the deflecting fields along the path traversed by the sodium atoms, the beam was split into individual beamlets in each of which the sodium atoms were in the same hyperfine quantum state. The total number of beamlets depended on the nuclear spin of sodium, therefore, all they had to do, was count the number of beamlets observed at the detector. From this they could infer that the nuclear spin of sodium is 3/2, but for many months they did not communicate their findings, and the first experimental results were published only in March 1933. In the same year 1933, Stern and his group had measured the magnetic moment of the proton, which was found to be about 2.8 times larger than what Paul Dirac's 1928 theory seemed to predict. This unexpected result was in fact a major discovery. The discovery of the spin of the electron had been of first importance in obtaining an understanding of atomic structure. Likewise, it was expected that a knowledge of the magnetic moment of the proton would play a similar role in the field of nuclear structure.

The fundamental character of the measurements of Stern and his collaborators prompted Rabi to set up his own experiment to measure the proton's - as well as the deuteron's - magnetic moment. With two postdoctoral fellows, J. M. B. Kellog and Jerrold R. Zacharias, Rabi quickly began to set up an experiment at Columbia University to measure the magnetic moment of the proton, by applying the Breit-Rabi theory. Results published in 1934 indicated an even larger value than Stern's surprising result. Further attempts performed in 1936 utilised a new method with two deflecting magnets that each beam particle passed sequentially. After being deflected in the first inhomogeneous magnetic field, both fast and slow atoms would be refocused into the detector by the second inhomogeneous field, avoiding complications associated with the distributed velocities of the beam particles. Between the two deflecting magnets there was a new static, T-shaped field. Beams passing trough the static field saw the equivalent of a rotating, or oscillating, magnetic field, which exerted a tipping force on the magnetic moment making it flip from one orientation to another when the apparent field had an angular velocity approximately equal to the Larmor precession frequency of the magnetic moment about the magnetic field. The study of these stimulated transitions between magnetic states of the hydrogen atom allowed, for the first time, to determine that the magnetic moments of the proton and deuteron are positive. The effect of this new arrangement was that it greatly improved the experimental results, reducing the uncertainty in the measured value of the proton's magnetic moment from 10 percent to 5 percent and 4 percent instead of 26 percent for the deuteron. But not only did these results provide better values and the signs of the moment, but also the magnetic moment of the neutron.

The Magnetic Resonance Method Throughout most of the 1930s Rabi and his collaborators, which by this time included also Polykarp Kusch, Sydney Millmann and Norman Ramsey, continued to investigate the first two isotopes of the hydrogen atom. In planning a third experiment, an apparatus very similar to that used in the preceding experiment was designed, but in a somewhat modified form. The two strong inhomogeneous deflecting fields were again set up to deflect beam particles in opposite directions, and the field strength of the second magnet was set to exactly undo what the first magnet did, that is to refocus the beam particles into the detector. If these two fields alone were acting on the beam, the number of atoms detected would be the same as if there were no fields present, because the second field would compensate exactly the action of the first field. The real novelty of this experiment was that the third simple static T-field was supplemented by a weak field component superimposed at right angles to the strong constant homogeneous field and oscillating at an adjustable radio frequency. This oscillatory component could change the orientation of the precessing atoms inducing transitions (flipping over) of the magnetic moments just before they entered the second constant inhomogeneous field.

In full analogy to the resonance absorption of visible light, transitions to different quantum states could occur from one Zeeman hyperfine level to another if the alternating field satisfied Bohr’s frequency condition for the energy difference between the two levels. However, instead of optical frequencies one dealt here normally with frequencies in the radio range, because the differences between the energy levels are very small. Every molecule saw many cycles of the same frequency and the probability of a transition was thus enhanced. When the Larmor precession frequency in the static field matches the frequency of the oscillating field, many atoms flip to another orientation and miss the detector. In this case the detector registers a marked resonance minimum, the frequency position of this minimum being determined with the extraordinary precision achievable with the radio frequency gauge. When the Larmor frequency is no longer in resonance with the frequency of the oscillating field, the atoms are all refocused into the detector, and the signal is once again large. This was the core of the magnetic-resonance method, the most significant improvement in molecular and atomic beam techniques, which clearly offered unprecedented accuracy in establishing radio relations with the world of the electron and of the atomic nucleus. Its most direct application was the measurement of nuclear magnetic moments. The basis for this is the resonance condition f=(μH)/Ih, in which f is the frequency of precession of the axis of nuclear spin in a magnetic field of strength H, and μ is the magnetic moment of the nucleus. The number I is the nuclear spin quantum number, an integer or half-integer, and h is Planck’s constant. The frequency of precession, once it is detected, is easily measured with high accuracy, and thus one can determine the quantity μ/Ih, and the magnetic moment can be found if the spin is known. Therefore, if the frequency of the oscillating field is slowly varied, a sharp decrease (the resonance phenomenon) occurs in the number of atoms arriving at the detector when the frequency of the field equals the Larmor frequency. Each such resonance then gives a value of the ratio μ/Ih and, hence, of the magnetic moment.

The first nuclear magnetic resonance curve was sent to Physical Review on January 15, 1938. Measurements on hydrogen with the resonance method continued in the late spring of 1938. As predicted, two strong resonances were observed with the molecule HD, one of which was associated with the proton, the other with the deuteron. Both these resonance absorptions made it possible to determine the magnetic moments of both the proton and the deuteron with improved precision. However, both the molecules H2 and D2 were presenting a pattern of different absorptions, instead of the single, strong narrow resonance the group expected. A new apparatus revealed the details of the multiple resonance pattern, but theory did not account for the obtained data, and Rabi soon realised that this might be due to the existence of another unsuspected property of the deuteron: a small but finite electrical quadrupole moment, which is a measure of lowest-order departure from a spherical charge distribution. This far-reaching discovery, announced in 1940, was quite a surprise. It immediately obliged theoreticians to renounce the central forces assumed to bind the neutron and proton together and to admit that nuclear forces are much more complex than the first nuclear models of the early 1930s had assumed. After the gap in the annual succession of Nobel Prizes, due to the Second World War, it was not until the autumn of 1944 that the Royal Swedish Academy of Sciences announced that for 1943 the prize would be awarded to Otto Stern, "for his contribution to the development of the molecular ray method and his discovery of the magnetic moment of the proton", and that for 1944 to Isidor Rabi, "for his resonance method for recording the magnetic properties of atomic nuclei". After World War II, nuclear magnetic resonance (NMR) became a workhorse for physical and chemical analysis. Still later, Rabi's discovery was extended to Magnetic Resonance Imaging (MRI), a powerful medical diagnostic tool, which is now used in medical centres the world over. In subsequent decades, the molecular beam method has been widely adopted by the physics and physical chemistry communities world wide, and about 20 Nobel Prizes were awarded for work based on the molecular beam method; among them there were Kusch and Ramsey, two of Rabi's former collaborators.

Bibliography Krige, J. (2005) Isidor I. Rabi and CERN. Physics in Perspective 7: 150-164 Rabi, I. I., Interview by Thomas S. Kuhn, December 8, 1963. Niels Bohr Library \& Archives, American Institute of Physics, College Park, MD USA, http://www.aip.org/history/ohilist/4836.html Rigden, J. S. (1983) Molecular Beam Experiments on the Hydrogens during the 1930s. Historical Studies in the Physical Sciences 13(2): 335-373 Rigden, J. S. (2008) Rabi, Isidor Isaac. In Complete Dictionary of Scientific Biography. Vol. 24. Detroit: Charles Scribner's Sons, Detroit. pp. 191-197. Gale Virtual Reference Library http://go.galegroup.com/ps/i.do?id=GALE%7CCX2830906032&v=2.1&u=mpi_vb&it=r&p=GVRL&sw=w&asid=49bfc365b28fab40b3d973344d8135cf Toennies, J.P. et al. (2011) Otto Stern (1888-1969): The founding father of experimental atomic physics. Annalen der Physik 523(12):1045-1070 Wasson T. (ed) (1987) Rabi, I. I. In Nobel Prize Winners, H. W. Wilson Company, New York, pp. 847-849

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Otto Stern (1888-1969): The founding father of experimental atomic physics

2011, Annalen der Physik

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Molecular Beams in Physics and Chemistry

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Motivated by his interest in thermodynamics and the emerging quantum mechanics, Otto Stern (1888–1969) launched in 1919 his molecular beam method to examine the fundamental assumptions of theory that transpire in atomic, molecular, optical, and nuclear physics. Stern’s experimental endeavors at Frankfurt (1919–1922), Hamburg (1923–1933), and Pittsburgh (1933–1945) provided insights into the quantum world that were independent of spectroscopy and that concerned well-defined isolated systems, hitherto accessible only to Gedanken experiments. In this chapter we look at how Stern’s molecular beam research came about and review six of his seminal experiments along with their context and reception by the physics community: the Stern-Gerlach experiment; the three-stage Stern-Gerlach experiment; experimental evidence for de Broglie’s matter waves; measurements of the magnetic dipole moment of the proton and the deuteron; experimental demonstration of momentum transfer upon absorption or emi...

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The history of Otto Stern’s pioneering measurement of the Maxwell-Boltzmann velocity distribution of a Silver atomic beam performed 1919 in Frankfurt is described. It is shown how Albert Einstein influenced Stern in his research. This experimental apparatus is not any more existing; therefore it was reconstructed in the workshops of the Physics faculty of the Goethe University in Frankfurt. The experimental verification of Stern’s results was finally achieved by a team of Frankfurt high school students (Gymnasium Riedberg) under the supervision of their teachers Axel Gruppe and Simon Cerny. By fighting against a number of difficulties, they succeeded to get the reconstructed apparatus started and were able to reproduce the results from the early experiments of Stern.

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The two years that Otto Stern spent with Albert Einstein in Prague and Zürich, between the spring of 1912 and the spring of 1914, can be viewed as his apprenticeship in theoretical physics. This chapter describes that formative phase in Stern’s scientific career, prior to his emergence as one of the greatest innovators in experimental physics.

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Abstract: A complete history of early atomic models would fill volumes, but a reasonably coherent tale of the path from mechanical atoms to the quantum can be told by focusing on the relevant work of three great contributors to atomic physics, in the critically important years between 1904 and 1913: JJ Thomson, Ernest Rutherford and Niels Bohr. We first examine the origins of Thomson&#x27;s mechanical atomic models, from his ethereal vortex atoms in the early 1880&#x27;s, to the myriad&quot; corpuscular&quot; atoms he proposed following the discovery ...

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A Stern–Gerlach experiment constructed in the spirit of the original experiment is reported that demonstrates spatial quantization for a beam of silver atoms directly to the unaided eye. Splitting of the atomic beam is seen both parallel to the magnetic field direction, as in the ‘‘usual’’ experiment, and in a direction mutually perpendicular to the field and atomic trajectory, which is a new transverse Stern–Gerlach effect. Splitting of up to 4 mm can be recorded visually. Calculations based on the force components acting on the atomic beams reproduce the experimental results within the experimental accuracy of 60.3 mm. © 2003 American Association of Physics Teachers.

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Part of the Einstein exhibition.

The Great Debate

With the turn of the 20th century, the field of physics underwent two major transformations, roughly at the same time. The first was Einstein's General Theory of Relativity, which dealt with the universal realm of physics. The second was Quantum Theory, which proposed that energy exists as discrete packets—each called a "quantum." This new branch of physics enabled scientists to describe the interaction between energy and matter down through the subatomic realm.

Einstein saw Quantum Theory as a means to describe Nature on an atomic level, but he doubted that it upheld "a useful basis for the whole of physics." He thought that describing reality required firm predictions followed by direct observations. But individual quantum interactions cannot be observed directly, leaving quantum physicists no choice but to predict the probability that events will occur. Challenging Einstein, physicist Niels Bohr championed Quantum Theory. He argued that the mere act of indirectly observing the atomic realm changes the outcome of quantum interactions. According to Bohr, quantum predictions based on probability accurately describe reality.

Niels Bohr and Max Planck, two of the founding fathers of Quantum Theory, each received a Nobel Prize in Physics for their work on quanta. Einstein is considered the third founder of Quantum Theory because he described light as quanta in his theory of the Photoelectric Effect, for which he won the 1921 Nobel Prize.

May 15, 1935: The  Physical Review  publishes the Einstein, Podolsky, and Rosen (EPR) paper claiming to refute Quantum Theory.

Newspapers were quick to share Einstein's skepticism of the "new physics" with the general public. Einstein's paper, "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?" prompted Niels Bohr to write a rebuttal. Modern experiments have upheld Quantum Theory despite Einstein's objections. However, the EPR paper introduced topics that form the foundation for much of today's physics research.

Einstein and Niels Bohr began disputing Quantum Theory at the prestigious 1927 Solvay Conference, attended by top physicists of the day. By most accounts of this public debate, Bohr was the victor.